Apparatus for hydrate-based desalination using compound permeable restraint panels and vaporization-based cooling

ABSTRACT

Desalination apparatus based on porous restraint panels fabricated from a number of different layers of metal, thermoplastic, or other substances are used as sophisticated heat exchangers to control the growth of gas hydrate. The gas hydrate is produced after infusion of liquid hydrate-forming material into water to be treated, which liquid hydrate-forming material can also be used to carry out all the refrigeration necessary to cool seawater to near the point of hydrate formation and to cool the porous restraint panels. Hydrate forms on and dissociates through the porous restraints. The composite restraint panels can also be used in gaseous atmospheres where, for instance, it is desired to remove dissolved water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/785,033filed Apr. 13, 2007, the contents of which are incorporated byreference. That application is based on and claims the priority benefitof U.S. provisional application 60/811,760 filed Jun. 8, 2006, thecontents of which are also incorporated herein by reference.

GOVERNMENTAL SUPPORT AND INTEREST

This invention was made with Governmental support under ONR CONTRACT:N00014-04-C-0237 dated Jun. 4, 2004, and amended May 18, 2005, andissued by the Office of Naval Research. The Government has certainrights in the invention.

FIELD OF THE INVENTION

In general, the invention relates to desalination and watertreatment/purification. More particularly, the invention relates toapparatus and methodologies for permeable restraint-supported hydrateformation and dissociation used to achieve such desalination and watertreatment.

BACKGROUND OF THE INVENTION

Gas hydrate forms when a hydrate-forming gas such as methane or any ofthe hydrocarbon gases, carbon dioxide, or chlorine, amongst others, isintroduced into water (or where water vapor is introduced intohydrate-forming gas) to appropriate concentrations under suitableconditions of pressure and temperature and in suitable manner so thathydrate crystal nucleation and growth take place. Hydrate may also beformed when an appropriate hydrate-forming gas and water solution thatis at pressures suitable for hydrate formation is chilled. Hydrategrowth is not only dependent on sufficient pressures and temperatureconditions; proper levels of concentration of the dissolvedhydrate-forming materials (HFM's) must also be maintained.

So far as is known to me, previous attempts by others to use hydrate forseawater desalination and water treatment, which attempts introduced gasdirectly into the water to be treated (henceforth referred to asseawater, although other water may be treated), always ultimatelyresulted in the production of a slurry formed from tiny shards ofrelatively pure hydrate. (The shards were formed when hydrate shellsformed around HFM gas bubbles would fracture.) Thus far, it has not beenpossible to purify such slurries sufficiently for the direct injectionof HFM into seawater to be a viable process for commercially producingfresh water because too much low-salinity water had to be consumedwashing the slurry.

In contrast, growth of larger masses of solid hydrate as described inU.S. Pat. No. 6,890,444, which facilitates separation of the hydrate andthe residual, enhanced-salinity water, requires that HFM concentrationas well as pressure and temperature in the water mass in which it isdesired to nucleate and grow hydrate be maintained at appropriatelevels. Published hydrate growth models and experimentation that havebeen described in Chapter 2 of Max et al., “Economic Geology of NaturalGas Hydrate,” Springer, Berlin, Dordrecht, 2006, demonstrate that growthof solid hydrate can be best achieved by maintaining an appropriatelyhigh concentration of HFM dissolved in water and then loweringtemperature. Seawater desalination, for instance, can take place where ametered supply of dissolved HFM can be brought into the presence ofhydrate in a seawater matrix, as is described in U.S. Pat. No.6,890,444, and where pressure/temperature conditions remain suitable forhydrate growth, even where such suitable conditions are very localized.

As taught in U.S. Pat. Nos. 7,008,544 and 7,013,673, the contents ofwhich are incorporated by reference in their entirety, gas hydrate canbe induced to form in an oceanic or artificially pressurized environmentin which pressure and HFM concentration are suitable for hydrate to formbut in which temperature is generally too high for it to do so. Inparticular, within environments such as these, hydrate can be induced toform on a surface (also referred to as a “restraint”) by chilling thesurface so that the pressure and temperature conditions for forminghydrate are produced locally on and near the surface. The surface mayhave pores or penetrations which constitute porosity of the restraint.Hydrate will grow on and outwardly away from the surface when thechilled surface is immersed in a body of water under suitable pressureand having appropriate concentrations of hydrate-forming material (HFM)dissolved therein (or in a gaseous atmosphere of HFM with appropriateconcentrations of water vapor dissolved therein). Lowering temperatureof the chilled surface causes hydrate to form on it and in its vicinity,thus filling the pores and blocking permeability.

In such processes, hydrate growth takes place through mass transfer ofreactants from the region of hydrate instability to the narrow region ofhydrate stability near the chilled porous restraint. The hydrate growthfront advances into the water (or gas in the case of a gaseousatmosphere) and away from the porous restraint as water immediately atthe hydrate face is cooled to the point at which hydrate is stable.Growth is sustained by the chilling of the porous restraint, whichcompensates for the heat of exothermic crystallization of the hydrate.

Sealing the pores of the restraint allows a pressure differential to beestablished and maintained across the restraint. In particular, loweringthe pressure of the environment on the side of the restraint across fromthe hydrate (the “downstream” side) causes the hydrate closest to theporous restraint to dissociate or melt, which allows water and gas thathave been contained in the solid hydrate crystal lattice to pass throughthe restraint into a collection region where they separate. The waterderived in this process is low in salinity and is collected andconcentrated for use. The process of water desalination through hydrateformation/dissociation can be steady-state, in which case hydrate growthand dissociation proceed simultaneously and at about the same rate, orcyclic, in which case there are alternating periods of predominantlyhydrate growth or predominantly hydrate dissociation.

SUMMARY OF THE INVENTION

The present invention significantly improves on the methodologies andapparatus taught in my previous, above-reference patents

In one aspect, the invention features a method for desalinating orotherwise purifying water to be treated using a hydrate-formingmaterial. The method includes introducing water to be treated into anenclosure containing one or more Hydrate Asymmetric Restraint Technology(“HART”) modules. Each of the HART modules includes one or more HARTrestraint panels, with pores extending from one major surface of therestraint panel to an opposite major surface of the restraint panel, andan internal chamber. In a first cooling process, the water to be treatedgenerally within the enclosure is cooled to a temperature that isslightly above a temperature at which hydrate of the HFM would form atpressure conditions existing within the enclosure; this first coolingprocess is effected by introducing HFM into the water to be treatedwithin the enclosure. In a second cooling process, water to be treatedthat is generally adjacent to the HART restraint panels is cooled to atemperature at which hydrate of the HFM forms at the pressure conditionsexisting within the enclosure; this second cooling process is effectedby refrigerating the HART restraint panels. As a result, hydrate of theHFM forms within the pores of the HART restraint panels, with sufficienthydrate being formed to fill and essentially seal the pores of the HARTrestraint panels. Downstream portions of the hydrate within the pores ofthe HART restraint panels are caused to dissociate, thereby releasingpurified water and HFM into the internal chambers of the HART modules,and the purified water is removed from the enclosure.

In specific embodiments, the enclosure may be located at depth within abody of water, with the pressure conditions within the enclosure beingcreated by the weight of water above the enclosure. In such case, theHFM may be delivered to the enclosure in liquid form by allowing it toflow from a self-pressurizing surface-level supply of HFM to theenclosure. Alternatively, the enclosure may be a pressure vessel, inwhich case the pressure conditions within the enclosure may be createdby pumping and/or vaporization of liquid-form HFM as it is introducedinto the water to be treated.

Furthermore, compressed, ordinarily gaseous HFM may be introduced intothe water to be treated in liquid form, in which case the first coolingprocess occurs as the HFM vaporizes and expands within the water to betreated. Alternatively, gaseous HFM may be introduced into the water tobe treated, in which case the first cooling process can occur as the HFMexpands within the water to be treated (although such cooling will be toa lesser extent than that which occurs when liquid-form HFM vaporizesand expands within the water to be treated). Ideally, sufficient amountsof HFM are introduced into the water to be treated to establish andmaintain saturation levels of HFM within the water to be treated withinthe enclosure.

Regarding the second cooling process, the HART restraint panels may berefrigerated by means of cooled liquid refrigerant circulatinginternally within cooling galleries within the HART restraint panels.Preferably, however, the HART restraint panels may be refrigerated bymeans of HFM passing internally through cooling galleries within theHART restraint panels. In the latter case, HFM is even more preferablyintroduced into the cooling galleries in liquid form and vaporizesinternally within the HART restraint panels—most preferably across arefrigerant distribution member from liquid HFM supply sides of thecooling galleries to gas sides of the cooling galleries. HFM that haspassed internally through the HART restraint panels is suitablyrecovered and also introduced into the water to be treated.

In addition to HFM that has been used to refrigerate the HART panels,HFM released into the internal chambers of the HART modules may also berecovered and recycled in further cycles of desalination orpurification. So, too, may HFM be recovered from the purified water andreused in further cycles of desalination or water purification.

In operation of the method, supplemental water to be treated may beintroduced into the enclosure to compensate for the purified water thathas been removed from the enclosure and/or to compensate for enhancedsalinity residual brine that has been evacuated from the enclosure. Thesupplemental water to be treated may be introduced into the enclosure ona generally continuous basis (i.e., continuously but at intervals orconstantly); alternatively, new water to be treated may be introducedinto the enclosure only after residual salinity within the enclosure hasreached a predetermined enhanced level of salinity been evacuated.

Preferably, HFM is recovered in gaseous form from within the internalchambers of the HART modules and/or in gaseous form after having beenused to refrigerate the HART restraint panels. The recovered gaseous HFMfrom either or both of these sources may be mixed with (e.g., dissolvedinto) water to be treated within the enclosure to help maintainsaturation levels of HFM within the water to be treated.

In another aspect, the invention features a method for desalinating orotherwise purifying water to be treated using a hydrate-forming material(HFM). The method includes introducing water to be treated into anenclosure containing one or more HART modules—each of the HART modulesincludes one or more HART restraint panels, with pores extending fromone major surface of the restraint panel to an opposite major surface ofthe restraint panel, and an internal chamber—and HFM is introduced intothe water to be treated within the enclosure. Water to be treated thatis generally adjacent to the HART restraint panels is cooled to atemperature at which hydrate of the HFM forms at pressure conditionsexisting within the enclosure; as a result, hydrate of the HFM formswithin the pores of the HART restraint panels, with sufficient hydratebeing formed to fill and essentially seal the pores of the HARTrestraint panels. This cooling is effected by refrigerating the HARTrestraint panels by passing HFM through cooling galleries extendinginternally throughout the HART restraint panels. Downstream portions ofthe hydrate within the pores of the HART restraint panels are caused todissociate, thereby releasing purified water and HFM into the internalchambers of the HART modules, and the purified water is removed from theenclosure.

Specific embodiments of this aspect of the invention may include one ormore of the features described above with respect to the first aspect ofthe invention.

In another aspect, the invention features apparatus for desalinating orotherwise purifying water to be treated. The apparatus includes anenclosure with one or more HART modules disposed therein. Each of theHART modules includes one or more HART restraint panels, with poresextending from one major surface of each restraint panel to an oppositemajor surface of each restraint panel, and an internal chamber formedtherein. Furthermore, each of the HART restraint panels has a series ofcooling galleries extending internally throughout it between the pores.A first conduit is arranged to supply hydrate-forming material to theenclosure, and a second conduit is arranged to remove purified waterfrom the internal chambers of the HART modules.

In preferred embodiments, the enclosure may be submersible andconfigured to be placed in pressure-equalizing communication withambient, sub-aquatic surroundings. Alternatively, the enclosure may be apressure vessel.

Preferably, the pores of the HART restraint panels tapernon-monotonically, e.g., they may have a bi-cone configuration.Furthermore, to facilitate such geometries, the HART restraint panelsare preferably composite restraint panels made from upper and lowerrestraint panel sections. Even more preferably the HART restraint panelseach include a refrigerant distribution member disposed between theupper and lower restraint panel sections, with microporous regions orvery tiny (e.g., on the order of about thirty to about eighty microns)holes disposed within the cooling galleries of the HART restraint panelsand dividing the cooling galleries into two sub-galleries.

In another aspect, the invention features a method for refrigerating aheat exchange panel. The method includes filling a series of firstsub-galleries extending internally throughout the heat exchange panelwith liquid refrigerant and vaporizing the liquid refrigerant across arefrigerant distribution member into a series of second sub-galleriesextending internally throughout the heat exchange panel.

In yet another aspect, the invention features a heat exchange panel. Theheat exchange panel includes an upper panel section joined to a lowerpanel section, with a series of cooling galleries defined between theupper and lower panel sections and extending internally throughout theheat exchange panel.

In preferred embodiments, the heat exchange panel may be a HARTrestraint panel, with pores extending through the heat exchange panelfrom one major surface thereof to an opposite major surface thereof.More preferably, the pores taper non-monotonically, e.g., they may havea bi-cone configuration. Even more preferably, the heat exchange panelmay include a refrigerant distribution member which divides the coolinggalleries into upper and lower sub-galleries. The refrigerantdistribution member may have microporous regions disposed within thecooling galleries, or it may have holes on the order of about thirty toabout eighty microns disposed within the cooling galleries.

Furthermore, the heat exchange panel may include a multitude of vorticalflow-inducing members—e.g., protrusions or dimples—distributed across amajor surface of the panel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in connection withthe Figures, in which:

FIG. 1 is a schematic diagram illustrating a submerged, open-oceanembodiment of water purification apparatus according to the invention;

FIG. 2 is a schematic diagram illustrating a submerged, oceanfloor-supported embodiment of water purification apparatus according tothe invention;

FIGS. 3 a and 3 b are a schematic perspective view and a schematicexploded view, respectively, of a HART module according to theinvention;

FIGS. 4 a and 4 b are a schematic perspective view and a schematic sideview, respectively, of a porous HART restraint panel used, for example,in the HART module of FIGS. 3 a and 3 b, illustrating the fundamentalHART process;

FIG. 5 is a schematic perspective view illustrating an array of HARTmodules used, for example, in the overall apparatus shown in FIG. 1,FIG. 2, or FIG. 31;

FIGS. 6 a and 6 b are a schematic exploded view and a schematicperspective view, respectively, of a portion of one embodiment of aporous HART restraint panel according to the invention;

FIGS. 7 a and 7 b are a schematic exploded view and a schematicperspective view, respectively, of a portion of another embodiment of aporous HART restraint panel according to the invention;

FIGS. 8 a and 8 b are schematic perspective views of an upper panelsection and a lower panel section, respectively, of one embodiment of acomposite porous HART restraint panel according to the invention;

FIGS. 9 a and 9 b are schematic perspective views of an upper panelsection and a lower panel section, respectively, of another embodimentof a composite porous HART restraint panel according to the invention;

FIG. 10 a is a schematic perspective view of a portion of a lower panelsection of another embodiment of a composite porous HART restraint panelaccording to the invention, and FIG. 10 b is an enlarged view of theupper left portion thereof;

FIGS. 11 a and 11 b are a schematic exploded view and a schematicperspective view, respectively, of a portion of another embodiment of aporous HART restraint panel according to the invention, including arefrigerant distribution member;

FIGS. 12 a and 12 b are, respectively, a schematic exploded view and aschematic assembled view—both in section—of a portion of a snap-togetherembodiment of porous HART restraint panel according to the invention,including a refrigerant distribution member;

FIG. 13 is a schematic perspective view illustrating a heat exchangepanel (e.g., a porous HART restraint panel) according to the invention,and

FIG. 14 is an enlarged, schematic side view of the boxed portion thereofillustrating vaporization of a liquid refrigerant across a refrigerantdistribution member;

FIGS. 15 a and 15 b are a schematic exploded view and a schematicperspective view, respectively, of a portion of another embodiment of aporous HART restraint panel according to the invention, including arefrigerant distribution member;

FIGS. 16 a, 16 b, and 16 c are a schematic plan view, a schematicsection view, and another schematic section view of a portion of theporous HART restraint panel shown, for example, in FIGS. 15 a and 15 b;

FIG. 17 is a schematic section view of another embodiment of a HARTmodule according to the invention;

FIG. 18 is a graph illustrating the variation of heat of vaporization ofliquid HFM (liquid CO₂) with depth;

FIG. 19 is a graph illustrating the variation of water temperature withdepth in an exemplary open-ocean environment;

FIG. 20 is a plot showing the CO₂ hydrate stability field inpressure/temperature space, along with the CO₂ liquidus, and control oftemperature in practicing the invention;

FIGS. 21 a and 21 b are schematic section views of two differentembodiments of apparatus according to the invention;

FIGS. 22 a and 22 b are schematic, detailed section views of twodifferent embodiments of porous HART restraint panels according to theinvention, illustrating the growth of hydrate in the pores thereof;

FIGS. 23 a and 23 b are schematic perspective views of portions of twodifferent embodiment of porous HART restraint panels according to theinvention, illustrating surface treatments to induce vortical flowacross the panels;

FIG. 24 is a plot, similar to that shown in FIG. 20, illustrating thedynamics of hydrate growth and dissociation according to the inventionin pressure/temperature space;

FIG. 25 is a plot illustrating the variation of the CO₂ hydrate phaseboundary with ambient water salinity in pressure/temperature space;

FIG. 26 is a plot illustrating the variation with depth (pressure) ofthe cooling potential obtained by vaporizing liquid HFM (liquid CO₂) inpressure/temperature space;

FIG. 27 is a schematic, detailed section view of a single pore within aporous HART restraint panel according to the invention, illustratinghydrate crystal kinematics;

FIG. 28 is a schematic perspective view of another embodiment ofapparatus according to the invention;

FIG. 29 is a schematic section view of the apparatus illustrated in FIG.28;

FIG. 30 is a plot illustrating the variation of fresh water that can berecovered by practicing the invention with different amounts of HFMrecycling; and

FIG. 31 is a schematic perspective view illustrating a pressurevessel-based embodiment of apparatus according to the invention.

(It should be noted that the various figures are schematic. Accordingly,dimensions and proportions may change between the various figures, evenwhere the same or similar components are shown in the various figures,for best illustration purposes. Accordingly, the text associated witheach figure should be consulted for proper understanding of the concepteach specific figure is intended to illustrate.)

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In general, according to the invention, gas hydrate (clathrate) isformed on permeable restraints broadly utilizing the approach of mypatents that are referenced above, which restraints are housed within anenclosure. The enclosure may be submerged within a naturally occurringbody of water (FIGS. 1 and 2), or it may be an artificially pressurizedvessel (FIG. 31). Hydrate-forming material (HFM) is infused into thewater being treated, and the permeable restraints are chilled by meansof an internal cooling system (illustrated and described in much greaterdetail below) so that hydrate forms on the surfaces of the permeablerestraints. Furthermore, utilizing the approach to desalination taughtin my above-referenced patents, the hydrate is caused to dissociatethrough the permeable restraints, thereby releasing HFM and relativelypurified water. The HFM may be captured for further infusion into waterbeing treated, and the purified water is gathered for consumption(possibly after further treatment).

Turning now to the figures, as shown in FIG. 1, in an open, mobilemarine installation, apparatus according to the invention is suspendedin the sea or other body of water 101. A preferred embodiment of suchapparatus is suspended from a floating platform 28 such as a ship, asemi-submersible platform similar to those used in the energy industry,a barge, or other floating platform. The floating platform may bemoored, drifting, or slowly underway while the apparatus is deployedbeneath it.

The invention includes apparatus for forming localized gas hydrate(which apparatus will be described in detail below) housed within anenclosure 8. (Reference numeral 8 will be used throughout to designatean enclosure in which hydrate forms and desalination takes place, evenwhere the specific embodiments of such an enclosure so designated areslightly different from each other.) The enclosure 8 is essentiallysolid on its sides 12, but it has openings for the introduction ofseawater on its top 18 and for the ejection of residual,enhanced-salinity water on its bottom 24. The openings may be valves orslots, either with preset widths or of variably controlled width, havingmotorized controls (not shown). The openings in the top and bottom ofthe enclosure 8 allow water to flow through the enclosure. In apreferred mode of operation, addressed in greater detail below, watermovement through the enclosure 8 is driven entirely by gravity. In othermodes of operation, water movement may be forced by the use ofimpellers, propellers, or other means of causing water flow.

The enclosure is connected to the surface by pipes 36, 42. One pipe 36provides a means for pumping liquid HFM from a tank or tanks 32 on theplatform to the enclosure 8, while the other pipe 42 carries lowsalinity water from the enclosure 8 to another tank 48 in the platform28. Other connections (not shown) between the surface and the enclosure8 may include sealed electrical wiring to provide power for sensors,electromechanical controls, communications cables, and provision forhydraulic systems (not shown) such as pumps. Either or both of the pipescan have adequate strength to support the weight of the enclosure,although a separate structural cable (not shown) may be used instead tosupport the suspended weight. Although the pipes might be assembled fromessentially straight segments during immersion of the enclosure 8 intothe sea, a preferred embodiment includes flexible pipes that can beunreeled from a winch drum within or on the deck of the ship orplatform. Such configuration allows for rapid deployment and initiationof desalination operations. It is also optimal for retrieval of theenclosure for maintenance, cleaning, or extraction in the event ofimpending severe weather conditions, when the floating platform may needto seek shelter at short notice.

In a fixed marine installation shown in FIG. 2, the enclosure 8 issecured in a mounting 52 (not shown in detail) in seawater 101, affixedto the seafloor adjacent to land 102. In this installation, the HFMsupply pipe and the low salinity water delivery pipes (shown as a singleline 59) may be rigid or flexible, or a combination thereof. The pipesdo not have to have sufficient strength to support the enclosure and/orthere is no need for a separate support cable or other apparatus tocarry the weight of the enclosure 8 from above. Although it is likelythat the enclosure 8 will be installed on a fixed mounting 52, theenclosure 8 could also be installed on a sliding mounting (notillustrated) that would allow the enclosure to be pulled at leastpartially up and let down the slope 55. This affords flexibility byallowing the installation to be operated at different water depths whilestill affixed securely to the seafloor.

In the fixed installation, a facility 65 on land for providing the HFMand for receiving the low salinity water normally controls operation ofthe system and passes the water on 70 to conventional treatmentfacilities (not shown). Because the pipes and control systems emanatingfrom the land installation can be relatively narrow in cross-section,they can be contained in a laterally drilled hole 75. Such a hole isrelatively easy and inexpensive to construct from the facility 65 to asub-marine junction facility 80, which can be installed on the seafloorbelow normal wave base for its physical security. Drilling the pipecourses to connect the sub-marine junction facility 80 with the surfaceinstallation 65 can be done much less expensively than tunneling.

A fixed installation is more secure than an open, mobile installationbecause it has a relatively small footprint, and the entire processsupply and control apparatus is deeply buried. The fixed marineinstallation cannot be easily approached or interfered with withoutsubstantial diving equipment. Moreover, inclement weather would be muchless likely to shut down operation of a fixed installation. Also,operation of a fixed installation would almost certainly require fewerstaff—the costs of ships and crews would not be required for a fixedinstallation—and thus be less expensive, although its capital cost mightbe greater than a mobile installation's capital cost.

With both the mobile and fixed marine installations, operation of theinvention utilizes natural pressurization by disposing the hydrateformation enclosure 8 at a depth in the body of water being treatedwhere pressure is high enough to hydrate to form. Any of the specificembodiments of enclosures 8 and apparatus contained therein may be usedin either a mobile or a fixed marine installation.

In either type of installation, the primary purpose of the enclosure 8is to restrict the flow of water in which the desalination and watertreatment apparatus is immersed, thus providing a defined, confined orpseudo-confined volume of water to be treated. Although the enclosure 8houses and physically protects the actual desalination apparatus, itfunctions in a manner that is integral to the desalination process ofthe invention. Because water inside the enclosure 8 is to be cooledaccording to the invention, as explained in greater detail below, theenclosure may be insulated to some degree to minimize heat transfer fromthe warmer seawater outside the enclosure to the water being treatedwithin. The size and shape of the enclosure 8 may vary, but it will begenerally rectilinear in form (although the external form can bemodified to fit any mounting system for either a mobile (FIG. 1) orfixed (FIG. 2) desalination installation).

Further description of the manner in which the enclosure is utilized inthe present invention is presented below, along with a more detaileddescription of other components of apparatus according to the invention.

As explained above in the Background section, in the above-referencedU.S. Pat. Nos. 7,008,544 and 7,013,673, hydrate is formed on (oraccumulates against or within) a permeable restraint or support member,and then is caused to dissociate through it from one side of therestraint to the other. I have termed the fundamental technologicalconcept that is the subject of those patents as well as the inventiondisclosed herein “HART,” for Hydrate Asymmetric Restraint Technology.

The restraints that were taught in my previous work—whether planar orcontoured—were relatively simple in configuration, providing essentiallya permeable plate (whether a flat plate or a curved plate) withcontinuously tapering pores extending from one surface of the restraintto the opposite surface of the restraint and internal cooling channelsextending throughout the restraint (e.g., internally within therestraint) between the pores. The present invention, in contrast, hasdeveloped the relatively simple configuration of those HART restraintsinto a far more sophisticated configuration, which allows the restraintsto be used as sophisticated heat exchangers as well as support surfaceson which hydrate forms. In particular, according to this invention,restraint panels are built up from sub-components, and pairs ofrestraint panels are joined via frames to form enclosed boxes orvolumes. I refer to the boxes as “HART modules.” A number of HARTmodules may be joined together within an enclosure 8, as will bedescribed in greater detail below. (Although it is preferable for eachHART module to have a pair of restraint panels to maximize the surfacearea on which hydrate formation and dissociation take place, it issufficient for purposes of the invention for a HART module to includejust one restraint panel.)

As shown in FIGS. 3 a and 3 b, a preferred embodiment of a basic HARTmodule 301 may be constructed as a relatively narrow, tabular orbox-shaped body with a porous restraint panel 304 forming each of thefront and back surfaces (i.e., the major surfaces) of the module 301.(The various embodiments of restraint panels illustrated and describedhereinbelow will all be labeled and referred to with reference numeral301 for convenience and simplicity, even where the specific internalgeometries differ slightly in various embodiments.) The porous restraintpanels are fitted into a frame 312, and the seams are sealed such thatgas and liquid will not pass through the seams. When assembled into theframe, the porous restraint panels may be flush with the frame, as shownin FIG. 3 a, or they may be inset or protrude slightly relative to theframe. A narrow chamber 319 is formed inside the frame, between the twoporous restraint panels. (See, also, FIG. 17.) The width of the chambermay be fixed by spacers (not shown) that separate the two porousrestraint panels when they are fully inserted into respective mountingcutouts in the frame 312. The spacers preferably separate the panels byabout no more than one inch (2.54 cm). If the internal chambers haverelatively large volume, buoyancy of the HART module—as explained below,there will be gas and liquid inside the chamber (which is submerged inwater) during operation of the invention—will vary over a greater rangethan if they have relatively small volume. Therefore, relatively smallerchamber volumes are preferred to provide more stable, predictablebuoyancy characteristics.

Further understanding of the components and operation of the HARTmodules 301 will be facilitated by a general description of theprocesses or functions served by the restraint panels 304. Specifically,there are inter-related processes of controlled chilling of the waterbeing treated; localized controlled growth of gas hydrate; and localizeddissociation of gas hydrate, so that the water bound in the hydrate maybe removed from the region in which it has formed. In general, asillustrated in FIGS. 4 a and 4 b, solid hydrate 306 (whose positionagainst the restraint panel 304 is indicated schematically by verticallines in FIG. 4 a) is caused to form on one side of a restraint panel304, namely, its outer surface 308, i.e., the surface of the restraintpanel that is exposed to the water being treated. As described in muchgreater detail below, cooling passages or “galleries” extend internallythroughout the restraint panel 304, generally parallel with the outerand inner surfaces 308, 309, to form an internal cooling system. Meansfor introducing 321 and removing 325 the coolant/refrigerant into andout of the cooling system are also provided. Chilling of the face of therestraint panel, so that localized conditions suitable for hydrategrowth, is effected by circulating refrigerant through the coolingsystem. In a preferred embodiment, the coolant/refrigerant, which mostpreferably is HFM, enters the restraint panel in liquid form andvaporizes therein to chill the panel; alternatively, liquid refrigerant(e.g., ethylene glycol) that has been thoroughly chilled may becirculated through the cooling system, with the refrigerant bothentering and exiting the cooling system in liquid form.

Once solid hydrate 306 has formed on the porous restraint panel, thepores of the restraint panel become clogged with the hydrate. The phaseboundary for hydrate grown on the upstream side of the restraint (FIG. 4b) is located near or at the location of contact between the growinghydrate and the water it is growing into, i.e., the hydrate/waterinterface, and the temperature of the system is lowest at the surface ofthe porous restraint panel. Because the thermal conductivity of hydrateis low, there is a temperature gradient through the hydrate mass, withtemperature normally increasing with distance from the surface of theporous restraint panel. Therefore, hydrate will grow on the upstreamside of the porous restraint panel, outwardly into the water beingtreated, so long as the temperature at the region of contact betweenhydrate and water is low enough, so long as there is an ample supply ofreactants (water and HFM, as indicated on FIG. 4 b by arrows in theupstream region), and so long as the water/hydrate interface is withinthe field of hydrate stability.

Hydrate that fills the pores of the restraint panel is exposed, throughthe pores, to the pressure conditions of the downstream region and canbe maintained at pressures and temperatures that are different fromthose at the hydrate growth front 308 because the hydrate forms apressure seal. Therefore, in order to dissociate the hydrate so that itsconstituent HFM and water can pass through the restraint panel and intothe region on the downstream side of the restraint panel, pressure onthe downstream side of the restraint panel is lowered. (Additionally, ifdissociation proceeds rapidly, heat may have to be added to the localsystem to prevent the temperature from falling below 0° C., i.e., toprevent water ice from forming.) When pressure in the downstream regionis lowered (and temperature is raised, if necessary/desired), thephysical conditions for hydrate instability and dissociation can becreated. Therefore, a second phase boundary 310 that is subject todifferent pressure/temperature conditions than those that exist at thehydrate growth front 308 can be established on the hydrate that isadjacent to the surface of or within the pores of the restraint panel.The conditions at this second phase boundary 310 can be maintained sothat the inner hydrate surface remains in a condition of instability dueto lower pressure to which it is subjected. Thus, by reducing pressurein the narrow chamber 319 within the overall HART module 301, whichchamber is the downstream region and which reduced pressure acts on thehydrate through the pores in the restraint panel, the portion of thehydrate that is exposed to that reduced pressure is caused to dissociateback into its constituent HFM 330 (typically in gaseous form) and water339. That water and HFM pass through the restraint panel 304 and intothe chamber within the HART module and are removed from the HART moduleby means described below. By exposing the hydrate to high pressure(i.e., pressure within the hydrate phase boundary for the temperature ofthe restraint) on the upstream side and relatively low pressure (i.e.,pressure outside the hydrate phase boundary) on the downstream side,hydrate can be formed and dissociated simultaneously on opposing sidesof its physical mass.

(Because the water differs in salinity, temperature, and gas content atdifferent points in the processes that are described herein, I refer toit using different terminology according to the point in the processbeing described. “Seawater” is the source water, regardless of itssalinity or gas content. It is the water from which bulk extraction ofwater molecules is achieved. “Captured water” is that water which hasbeen extracted from the seawater by incorporation into gas hydratethrough crystal growth processes. “Converted water” is water that isderived from or released upon the dissociation of hydrate. Convertedwater may contain some gaseous HFM that has also been released upondissociation of the hydrate. “Recovered water” refers to water that hasseparated from the released gaseous HFM and that has been removed to thepipe assembly associated with recovering water from the apparatus.“Produced water” refers to water that has been removed from thedesalination apparatus and water recovery pipe assembly and that isavailable at the surface for further processing and/or transport tomarket.)

Referring back to FIGS. 3 a and 3 b, each HART module 301 has fourseparate ports, two of which (321, 325) are provided for operation ofthe refrigeration systems of the porous restraint panels and two ofwhich (330, 339) are provided for collection of gas and water releasedby dissociating hydrate, which gas and water separate naturally. Aliquid refrigerant inlet 321 and its control sensors and valves 323 areprovided in the frame. Similarly, a refrigerant outlet 325 and controlsensors and valves 327 are provided on the frame. The valves 327 areconfigured for the specific nature of the refrigerant at its exit, viz.,gaseous or liquid. The water separation and collection process alsorequires at least two outlets—one for gas in the upper part of themodule and one for water in the lower part of the module. A gas outlet330 and its control sensors and valves 333 are provided in the upperpart of the module frame. A recovered water outlet 339 and controlsensors and valves 345 are provided in the lower part of the moduleframe. Preferably, a water dump valve 341 that is controlled by thevalves and sensors 345 is also provided in order to exhaust water thatmay have become contaminated.

The recovered water outlet 339 of each HART module 301 is connected to acollector system (not illustrated), whereas the water dump outlet 341allows water to be exhausted before it enters the collector system.Water may be exhausted by increasing gas pressure back in the modulesand expelling water, or it may be exhausted by continued production oflow salinity water though hydrate formation and dissociation. Each HARTmodule 301 also has provision for dumping water using gas flush byforcing gas into the module through the gas outlet 330 so that water canbe expelled, e.g., during startup or if a seawater breach through a poreresults in salinity rising to a level beyond that which is acceptable.(Some leakage of untreated seawater into the HART module 301 and mixingwith the product water is acceptable, so long as the overall salinity ofthe recovered water remains below the target product salinity.)

Each HART module 301 further has means for exhausting water using gasflushing so that water can be expelled from the module during systemstartup or if seawater breaches through a pore and causes salinity torise to a level within the module that is beyond that which isacceptable. Back-flushing the pores using either liquid or gas (when nohydrate is present) to remove unwanted particulate matter if flowbecomes restricted is also possible. (Some suspended and/or dissolvedmaterial in the product water is acceptable, so long as the overallsalinity of the recovered water remains below the target productsalinity.)

HART modules 301 are configured and arranged to operate in arrays withinan enclosure 8. In a preferred arrangement illustrated in FIG. 5, HARTmodules are placed alongside one another, oriented so that the internalchamber 319 of each module provides the greatest vertical height forseparation of gas from water that has been produced by dissociation ofgas hydrate through the porous restraint panels. Depending on desiredwater production, the size of the HART modules, and/or the size of theenclosure 8, there may be more than one row of HART modules arrayedwithin a given enclosure 8.

Restraints according to the invention (i.e., the porous restraint panels304) are referred to as “composite” because each is composed of two ormore component layers that are fabricated separately and then joinedtogether to make a tapered-pore porous restraint panel, whichfacilitates low-cost manufacturing, optimal refrigeration, and optimaltemperature control. The detailed geometry of the complexly shapedtapered pores will be discussed below, subsequent to more generaldescription of the embodiments and in relation to the process of hydrateformation and dissociation. By fabricating the porous restraint panelsin more than one section, complex internal geometries for open coolingchannels or “galleries” can be achieved that cannot be practicallyproduced in any other way. These internal galleries can be formed bymachining or other cutting process, in which case the restraint panelsare referred to as “thick wall” because they must be machined. “Thinwall” composite restraint panels, on the other hand, can be made bystamping, extruding, or some other industrial process such as directforming of a polymer. Thin wall restraint panels will have thinnertapered pore wall thicknesses and will almost certainly be lessexpensive to manufacture than thick wall restraint panels. Because boththe upper and lower sections of a porous restraint panel according tothe invention have a honeycomb structure, even in embodiments that mayhave very thin walls, the structures have considerable inherentstrength.

As shown in FIGS. 6 a and 6 b, the simplest embodiment of a compositeporous restraint panel 304 is fabricated in two sections. These twosections include an upper section 351, which faces the water mass beingtreated, and a lower section 353, the exposed face of which bounds theinternal chamber 319 of a HART module 301. (The upper section 351 isupstream with respect to the direction of movement of water in thehydrate desalination process, while the lower section 353 is downstreamrelative to the water flow through the porous restraint panel.) Theupper section 351, in which the hydrate is grown, and the lower section355, in which dissociation takes place, are joined together and formedinto a single unit (FIG. 6 b) via glue, resin, brazing or soldering,compression and shear or ultrasonic welding, or other processes known tofabrication industries.

In this embodiment, longitudinally extending refrigerant galleries 355are located inside of the upper section 351 so as to be internal withinthe porous restraint panel when the upper and lower sections areassembled together, as shown in FIG. 6 b. The refrigerant galleries areisolated from water; therefore, a gasket or sealant (not shown) may beprovided between the upper and lower sections to enhance sealing. Therefrigerant galleries 355 are shown in FIGS. 6 a and 6 b as triangularin cross-section for illustration only; it should be appreciated thatthey might have a cross-sectional shape that does not “follow” the shapeof the adjacent surfaces of the tapered pores on the side of the uppersection 351 that faces the water under treatment.

Furthermore, both the upper and lower restraint panel sections 351, 353have a plurality of tapered pores (illustrated by contour lines on thewalls of the pores) that are aligned with each other through thecomposite porous restraint panel. As addressed in more detail below,hydrate forms in the tapered pores in the upper restraint panelsection—thus, the pores in the upper restraint panel section constitutehydrate formation regions 358—and, in preferred embodiments of theinvention, dissociates in the tapered pores in the lower restraint panelsection. Notably, the mouths of the tapered pores in the lower section353 (i.e., the widest part of the pores) are wider than the outlets(i.e., the lowest parts) of the tapered pores in the upper section 351.This relationship results in a “double cone” profile of the compositetapered pores; in other words, there is a “step” or “jog” in theprofiles of the pores at the point 357 where the upper and lowersections 351, 353 are joined together, as is shown in FIG. 6 b. Thus,whereas the pores in the porous restraints shown in my earlier patentsdecrease monotonically in diameter, the overall or composite pores inthe composite restraint panels of this invention have a step at thejunction between the upper and lower sections, where the diameter ofeach composite pore expands slightly before decreasing again in thedirection from the upper section 351 to the lower section 353. (Thebenefits of such a configuration will be explained below in connectionwith a more detailed description of the hydrate formation anddissociation dynamics relating to this invention.)

If greater coolant capacity is required, additional longitudinallyextending channels 359 can be provided in the lower restraint panelsection 353, which correspond to and are aligned with the channels 355in the upper restraint panel section 351 as shown in FIGS. 7 a and 7 b.When the two sections are assembled together, the resultant refrigerantgalleries 365 are substantially larger than the ones in the embodimentshown in FIGS. 6 a and 6 b and the chilling potential is enhanced.

Furthermore, the refrigerant galleries can be formed as separate,longitudinally extending channels as shown in FIGS. 8 a and 8 b, inwhich case they each have a single entrance 367 and exit 369.Alternatively, the galleries may be cross-wise interconnected in alattice pattern, as shown in FIGS. 9 a and 9 b. In this latterembodiment, coolant galleries 267 which carry refrigerant completelysurround each pore. It should be appreciated that a wide variety oforientations of the channels or galleries that may vary from theorthogonal examples shown in FIGS. 8 a, 8 b, 9 a, and 9 b are possible,and the specific examples shown in FIGS. 8 a, 8 b, 9 a, and 9 b are usedfor the clarity of illustration only.

The inlets to and outlets from the refrigerant galleries in the porousrestraint panels are in communication with appropriate channels (notshown) within the frames 312. The frame channels, in turn, are incommunication with the HART module inlet ports 321 and outlet ports 325.Thus, liquid coolant—preferably liquid HFM—enters a given HART module301 through the inlet port 321; circulates within the refrigerationgalleries in both restraint panels 304 comprising the given HART module301 to chill the pores (and preferably vaporizes therein, as mentionedabove and as described in more detail below); and exits the given HARTmodule 301 in either liquid or, more preferably, gaseous form.

Providing chilling that is as uniform as possible throughout the porousrestraint panels is important for best hydrate growth. Best practice foreven cooling using recirculated fluid refrigerant is to circulate therefrigerant fast enough to keep overall temperature drop low. In orderto distribute refrigeration or chilling potential more equallythroughout the porous restraint panels, a refrigerant distributionmanifold 372 is provided in the lower member of a composite porousrestraint at the “head” of each gallery 375, as shown in FIGS. 10 a and10 b. The top wall of the distribution manifold 372 is constituted by asurface of the upper restraint panel member or by some other member (notshown) that limits the vertical extent of the manifold to just the lowerrestraint panel member. A means for regulating the flow of refrigerantfrom the manifold into the refrigeration galleries 375 (in an embodimentsimilar to that shown in FIGS. 6 and 7) may be provided between the endof each refrigerant gallery 375 and the distribution manifold 372. Inparticular, at this position, an end wall member 376 is provided, withan opening 379 that is designed to retard the flow of refrigerant intoeach of the galleries. This has the effect of more equally distributingrefrigerant into the refrigeration galleries.

The rate at which liquid refrigerant is introduced into the coolinggalleries (from the distribution manifold, if present) can be controlledby valves (not shown) at the HART module inlet 321 and/or the restraintpanel inlet 383 in the lower panel section that regulate(s) the flow ofliquid circulation refrigerant. Additionally, the rate of coolant flowthroughout the restraint panels can be regulated by controlling the“headspace” pressure in the refrigeration galleries 355 (e.g., FIGS. 6and 7) where a refrigerant vaporization system is used. Where pressuresin the galleries are maintained close to the liquid/gas vapor pressureof the liquid refrigerant (for any particular temperature), vaporizationcan be retarded and the chilling potential thus controlled. Morespecifically, varying pressure in the galleries by pumping at the outletcontrollers 327 of the HART modules 301 (FIGS. 3 a and 3 b) will controlthe rate of vaporization and hence temperature. Making the distributionof liquid refrigerant in the refrigerant gallery system of a porousrestraint panel as uniform as possible and controlling the rate ofvaporization of liquid refrigerant will help equalize chillingpotential, and hence the capacity for uniform chilling, across a givenporous restraint panel. Moreover, the manifold-to-refrigerant gallerysystem shown in FIGS. 10 a and 10 b, which distributes liquidrefrigerant, will also facilitate more uniform chilling potential incases where liquid refrigerant is vaporized.

The most uniform chilling potential can be obtained if vaporization canbe controlled so as to occur uniformly across the span of a porousrestraint panel. To this end, FIGS. 11 a and 11 b illustrate a compositepermeable restraint panel 304 in which large-volume refrigerationgalleries, similar to those illustrated in FIGS. 7 a and 7 b, areseparated into two sub-galleries (essentially, the channels 355, 399 inthe upper and lower panel sections 351, 353, respectively) by a“refrigerant distribution member” 391 that is disposed between the upperrestraint panel section 351 and the lower restraint panel section 353and across which liquid refrigerant vaporizes.

The larger holes 393 in the distribution member 391 coincide with thetapered pores extending through the upper and lower panel sections. Thelines of small holes 396 indicate diagrammatically the locations ofmicroporous areas or areas with very tiny holes in the refrigerantdistribution member 391, which microporous or very-tiny-hole areas allowrefrigerant to vaporize from the refrigerant supply galleries 399 in thelower restraint panel section into the cooling galleries 355 in theupper restraint panel section on a molecular basis. The refrigerantdistribution member 391 can be fabricated from a single material such asthin metal in which tiny holes (for instance, on the order of aboutthirty to about eighty microns) have been drilled (for instance, bylaser drilling) or from materials that have a microporous character.Alternatively, microporous membrane regions can be inset into thedistribution member at appropriate locations. The refrigerantdistribution member 391 is configured with microporous or very-tiny-holeareas distributed in essentially the same patterns as the channels inthe upper and lower restraint panel sections.

(Microporous membrane materials are semipermeable barriers. Themembranes are polymers and can have a variety of properties, e.g.,hydrophobic or hydrophilic. Such membranes can also be selective ornon-selective. Some polymers commonly used for microporous membranes arepolypropylene and polyolefin. Microporous hollow fiber membranestypically have outside fiber diameters of approximately 300μ and insidefiber diameters of approximately 200-220μ. Porosity typically rangesfrom approximately 25-40%.)

In this configuration, the galleries 399 in the lower restraint panelsection 353 (henceforth termed “refrigerant supply galleries”) arecontinuously filled with liquid refrigerant during operation of theapparatus. By lowering vapor pressure in the refrigeration galleries 355in the upper section 351 to below the vapor pressure of the gas/liquidtransformation point of the specific refrigerant, the liquid refrigerantin the refrigerant supply galleries can be caused to vaporize throughthe microporous regions 396 of the refrigerant distribution member 391and into the galleries 355 in the upper panel section 351, thusproviding cooling of the upper panel section 351 generally, i.e., on thegas side of the microporous membrane, because vaporization tends tooccur uniformly at all pores. This process may be referred to hereafteras “area vaporization.” (Cooling of the liquid refrigerant side of themicroporous membrane 391 in the lower section 353 is not desired;therefore, cooling is desirably restricted to just the upper panelsection 351, where it is desired.)

In general, refrigeration is important because the temperature of thesurfaces of the tapered pores of the restraint panels must be lowered toa point where hydrate is stable and will grow. Additionally, heatproduced by the exothermic formation of hydrate must be removed byrefrigeration so that the hydrate will continue to grow outwardly intothe seawater being treated. Area vaporization provides for the mostuniform distribution of chilling potential, especially where smallchanges in chilling potential are required over the entirety of a porousrestraint panel (or in any other vaporizer apparatus in which areavaporization may be employed). Uniform chilling potential and theability to control small temperature changes over a large area uniformlyare important because, in general, it is highly desirable for hydrate tobe grown at a uniform mass rate across the entire surface of a givenrestraint panel. Fine-scale temperature control and uniformity oftemperature over the entirety of the refrigerated area in which gashydrate is to be grown optimize pressure-sealing and water- andgas-removal from the upstream side of a porous restraint as per theprocess illustrated in FIGS. 4 a and 4 b.

To achieve a desired amount of chilling, liquid refrigerant isintroduced into the refrigerant supply galleries 399. Refrigeration iseffected by reducing the vapor pressure of gaseous refrigerant in therefrigeration galleries 355 to below the vapor pressure of thegas/liquid transition point (for any particular temperature) by pumping.When the vapor pressure in the refrigeration galleries is in the vaporphase for that refrigerant, vaporization takes place at or near therefrigerant distribution member 391 because gas is the stable phase atthe lower pressure. The lower the pressure that is maintained in therefrigeration galleries 355, the greater the cooling that is provided.In this manner, a particular range of chilling potentials andtemperatures can be maintained in the refrigeration galleries 355 bycontrolling the amount and rate of vaporization of refrigerant. Thispressure-based control of vaporization, and hence chilling potential,facilitates fine-scale control over and uniformity of temperaturethroughout the gallery system.

Extracting gas from the refrigeration galleries 355 using relativelysmall gas pumps—located anywhere in the gas extraction system betweenthe gas exhaust 330 on the HART module frame 312 and the gas collectorand handling system that is described below—controls vapor pressure inthe refrigeration galleries 355. Lowering gas head pressure faster toachieve greater cooling is achieved by faster pumping. Lowering pressurein the gas headspace more slowly reduces cooling. If the rate of pumpingfalls to too slow an extraction rate, the gas and liquid sides of therefrigerant distribution member 391 will tend to equilibrate inpressure.

Although the refrigerant distribution member 391 is generally locatedbetween the upper restraint panel section 351 and the lower restraintpanel section 353, the exact configuration and arrangement of thecomponents may be slightly more complex than that shown and describedabove, with beneficial results. For example, where the individualcomponents of the composite porous restraint panels are formed so thatthey partially nest within each other (as opposed to fitting togetherwith essentially flat junctions), the strength of the composite porousrestraint panel assembly can be increased. For example, as shown inFIGS. 12 a and 12 b, the cone walls of the tapered pores in the upperpanel section 351 fit into and extend slightly into the pores in thelower restraint panel section 353. Because the parts essentially snapinto each other, considerable strength is imparted to the assembledcomposite restraint panel. It should be understood that smalldifferences in the shape and arrangement of the members—for instance, toprovide a spacer against the lower member—may be implemented and arewithin the scope of the invention, although they are not described herein detail. If the joints between the components are essentially flat,shear along the boundaries of the components due to differentialpressure may cause small leaks of seawater or refrigerant; this can beavoided by making one or both of the upper and lower panel sectionsmechanically strong, but that may result in additional material andweight penalties. If, on the other hand, the upper and lower restraintpanel sections interengage with each other as shown in FIGS. 12 a and 12b, for example, a shear plane is less likely to form at the junctionbetween the upper and lower restraint panel sections and, accordingly,the panel will be less likely to leak. Additionally, one or more of theparts can be made thinner and lighter to take account of the increasedmechanical strength of an interengaging design.

In embodiments of the invention that use a three-component compositerestraint panel as shown in FIGS. 11 a, 11 b, 12 a, and 12 b, forexample, sealing the refrigeration system on the interior of thecomposite restraint panel from the water, gas, and hydrate phases on theoutside of the restraint panel is important. Therefore, the parts may bewelded, heat-bonded, or brazed/welded together; diffusion-welded in thecase of special metals such as titanium; or joined by other methods. Ifthe components are thermoplastic or if more than one material is used,glue or chemical bonding (which may also be used with metal) may also beused to join the parts together. Some composite porous restraint panelsmay include O-ring sealing, at least in part. Following assembly, theentirety of the three components are firmly joined together to form asingle, integrated restraint panel unit, with the liquid refrigerantsupply galleries 399 (as well as the gaseous refrigerant galleries 355)completely separated from the tapered pores.

The refrigerant distribution member 391 facilitates uniform distributionand control of chilling potential in the preferred embodiment of therefrigeration system. FIG. 13 illustrates in general, with internaldetails of the upper and lower restraint panel sections 351, 353 removedfor simplicity, how the refrigeration effect is achieved. As notedabove, the refrigerant distribution member 391 contains tiny holes(shown as uniformly distributed for simplicity). Liquid refrigerantoccupies the internal spaces within the lower restraint panel section353, while gaseous refrigerant normally occupies the internal spaceswithin the upper restraint panel section 351. The potential for chillingvia vaporization of the liquid refrigerant through the microporous holesin the refrigerant distribution member 391 is the same throughout theentirety of the gas spaces, in which pressure differences will rapidlyequilibrate.

A detailed, highly magnified schematic cross-section of the refrigerantdistribution member 391 (FIG. 14) illustrates the relationship betweenthe liquid and gaseous phases of the refrigerant. The refrigerationeffect—i.e., the degree of cooling—is managed by controlling pressure onthe gas side, e.g., by pumping as alluded to above. So long as vaporpressure on the gas side of the refrigerant distribution member 391 isin equilibrium with vapor pressure of the liquid refrigerant on theopposite side of the refrigerant distribution member 391 (i.e., in therefrigerant supply galleries 399), no mass transfer of refrigerantoccurs between the liquid and the gas sides of the distribution member391. (In this state, the vapor pressure of the system is the vaporpressure of the liquid refrigerant (for any particular temperature).)When pressure in the galleries in the upper restraint panel section islowered, however, the vapor pressure of the liquid refrigerant becomeshigher than the pressure on the gas side, and mass transfer through themicropores in the refrigerant distribution member 391—illustrated by thearrows in FIG. 14—takes place from the liquid side of the refrigerantdistribution member to the gas side of the refrigerant distributionmember. Thus, varying pressure on the gas side of the refrigerantdistribution member can be used to regulate vaporization of the liquidrefrigerant in the supply galleries 399, and hence cooling of the poresin the restraint panels in general.

Thus, the refrigerant distribution member 391 essentially functions as aphase partition and separates liquid refrigerant from gaseousrefrigerant. If little or no vaporization is taking place across therefrigerant distribution member 391 and the cooling potential is small,the liquid side of the refrigerant distribution member is maintained ator just above the vapor pressure of the gas/liquid transition, and thegaseous side of the refrigerant distribution member is held at or belowthe vapor pressure of the refrigerant. Increasing the pressuredifferential across the refrigerant distribution member will increasethe rate of vaporization and therefore increase the cooling potential.The micropores 396 are phyllic to the particular refrigerant (forexample, CO₂). Therefore, liquid refrigerant fills the micropores butsurface tension holds it within the micropores and prevents it frompassing as droplets or as a surface film onto the other side of therefrigerant distribution member, so long as the pressure differentialdoes not become excessive, as illustrated in FIG. 14. The gas side ofthe refrigerant distribution member may be coated with material such asa metallized surface that is phobic to the liquid refrigerant to keepthe liquid refrigerant from wicking onto the gas side of the refrigerantdistribution member. Vaporization is most efficient thermally when ittakes place entirely at the gas/liquid interface at the distributionmember 391 or within the gas chamber.

The chilling effect of vaporizing refrigerant across the refrigerantdistribution member is approximately the same throughout all of the gasspace within a given restraint panel because approximately the same rateand amount of vaporization takes place everywhere along the refrigerantdistribution member. This is because the liquid in the liquidrefrigerant supply galleries 399 and the gas in the refrigerantgalleries 355 constitute individual hydraulic systems that interact atthe refrigerant distribution member. Therefore, because the amount ofvaporization and hence refrigeration can be regulated closely by varyingpressure in the refrigeration galleries 355, the system response tochanging conditions and thermal demand required to chill the water andmaintain temperatures at which hydrate will form is greatly improved ascompared to the prior art.

During periods of higher chilling requirements, the vapor pressuredifferential between the liquid and gas sides of the refrigerantdistribution member may become great enough for liquid droplets to beejected through the refrigerant distribution member into the otherwisegas-filled refrigeration galleries 355, where they vaporize rapidly andcontribute to cooling. This is because the surface area for vaporizationof the liquid droplets is greater than that which is normally availableon the pore area of the refrigerant distribution member. This furtheraccelerates the chilling process and may be used as a technique toincrease cooling potential above that which is afforded by vaporizationat just the surface of liquid exposed in the micropores.

If a strictly circulating liquid refrigerant system is used forcooling/refrigeration instead of vaporization across the refrigerantdistribution member 391, larger holes may be provided in the refrigerantdistribution member (if one is even used at all) to allow the liquidrefrigerant to pass more easily and uniformly from the refrigerantsupply galleries 399 to the refrigerant galleries 355, since theinherent viscosity of liquid refrigerant will otherwise tend to restrictthe speed of its passage through the refrigerant distribution member391. The sole difference necessary for the refrigerant distributionmember to be used to equally distribute liquid instead of vaporizationis in the size of the distribution holes in the member; the overallbenefit of the refrigerant distribution member—equal distribution ofchilling potential—is, however, essentially the same.

If the LCO₂ is pure enough such that small particles do not exist withinit that could otherwise clog the vaporizer system, then LCO₂ can beemployed directly from a surface feed. This pass-through mode wouldinvolve no special recompression apart from that applied to gaseous CO₂recovered from the dissociated hydrate. It is likely, however, that avapor compression system using the same CO₂ may be employed. Once it ispurified of particulate matter, it can be cycled indefinitely, withtopping up only necessary to replace leaks. A purification/rechargesystem for the cooling of porous restraints by recirculating vaporcompression methods may be implemented in the immediate vicinity of theenclosure 8 or near the LCO₂ supply.

Reverting to more general considerations, a porous restraint panelaccording to the invention preferably has equal thickness for all porewalls—the controlling surfaces for heat exchange for the uniform growthof hydrate—and is as thin as possible so that heat transfer is enhanced.This class of porous restraint panel is referred to as “thin-wall” andcan be produced in limited numbers by machining or in larger numbers,for lower cost, by stamping (for instance, of thin metal sheets) orforming (for example, using a fluid-to-solid casting or extrusionprocess). Whereas machined components (e.g., as shown in FIGS. 6, 7, and11) begin with metal or other suitable material such as plastic orcomposite material similar to carbon fiber that is at least as thick asthe final restraint panel component, thin-wall porous restraint panelsmay be fabricated from thin sheets of material that are formed intocomplex, three-dimensional shapes. There are two basic ways to makethin-wall restraint panels: stamping and forming. In stamping, a sheetof metal such as titanium alloy that is very resistant to marinecorrosion (or other suitable material such as plastic) is stamped in adie by inserting the sheet of material in the die and pressing the sheetbetween the two dies. Formed components, on the other hand, can beextruded or cast.

FIGS. 15 a and 15 b illustrate a thin-wall upper restraint panel section351, a refrigerant distribution member 391, and a lower restraint panelsection 353 assembled into a single porous restraint panel (FIG. 15 b).The upstream face of the composite restraint panel faces outward from aframe in which the restraint panel is supported in much the same generalmanner as shown in FIGS. 3 a and 3 b for composite restraint panelshaving about equal thickness. Because thin-wall porous restraint panelscan be fabricated so that the pore walls are thin everywhere and haveabout the same thickness, the refrigerant galleries 355 and the liquidrefrigerant supply galleries 399 can constitute a substantial proportionof the overall volume of the whole thin-wall restraint panel, whichrenders thin-wall restraint panels highly efficient heat exchangers.Most important, however, is the fact that because the pore walls haveconstant thickness, the chilling effect can be uniform throughout all ofthe tapered pores in the upper section 351 adjacent to the refrigerationgalleries 355, and uniform chilling will optimize uniform hydrate growthin the pores.

FIGS. 16 a, 16 b, and 16 c illustrate (for just two rows of pores) anarrangement for mounting a thin-wall composite restraint panel, whichhas relatively thin edge margins. FIGS. 16 a, 16 b, and 16 c show anexample of an edge member 470—preferably made from plastic—that is castaround the edges of a single porous restraint panel 304. The edge member470 brings the thin edge margins of the restraint panel to about thesame overall thickness as the rest of the composite porous restraintpanel and lets the restraint panel be mounted in a HART module frame inessentially the same manner as discussed above and illustrated in FIGS.3 and 4 for a composite restraint panel having about uniform thickness.A section view taken through the centers of the tapered pores (FIG. 16b) shows the pores in the upper panel section 351 and the lower panelsection 353. The position of the refrigerant distribution member 391 isshown in solid black. In a section view that is taken between the mouthsof the tapered pores (FIG. 16 c), the thickness of the upper panelsection 351 and the lower panel section 353 are more apparent. Thedistribution member 391 is again shown in solid black.

Alternatively, in a preferred embodiment as illustrated in FIG. 17, twoporous restraint panels may be cast so that they are integral with aframe 473 (analogous to the frame 312 shown in FIGS. 3 a and 3 b),together with which they form a basic HART module (e.g., as discussedearlier in relation to FIGS. 3 a, 3 b, 4 a, 4 b, and 5). Spacers (notillustrated) can also be added within the internal chamber 319 duringfabrication to help maintain a constant width of the central chamber 319when pressure therein is reduced during operation of the apparatus ofthe invention. Additionally, in this configuration of a HART module, theframe itself does not contain refrigerant inlet ports and exhaust ports(e.g., as described above in connection with FIGS. 3 a, 3 b, 4 a, 4 b,and 5); rather, inlets 322 and outlets 326 to one or more locations oneach composite restraint are provided. (One inlet and outlet are shownper each composite porous restraint panel in FIG. 17, but this is forillustration purposes only.) Thus, the frame and refrigerantdistribution system can be implemented in a number of differentways—both through the frame and directly to each composite porousrestraint.

Furthermore with respect to this embodiment, the tapered pores 358 arepreferably staggered or offset with respect to each other so that eachHART module 301 can be made thinner. This arrangement also improves theoverall process of hydrate growth and dissociation in each tapered pore,which will be explained in greater detail below.

In all of the embodiments of porous restraint panels described herein,other apparatus may be added to achieve better thermodynamic performanceand to enhance liquid, gas, and water flows. For instance, where it isdesired to have more than one internal gallery system for chilling orfor carrying a sensor system (mainly pressure and temperature sensors),these may be implemented using another member in the composite restraintpanels. Furthermore, a layer of insulation may be added on the face ofeach of the porous restraint panels in order to prevent unwanted orexcessive hydrate from forming on the restraint panels in regions otherthan the tapered pores. Surfaces may also be treated, for instance, byanodizing or coating with corrosion-resistant materials such as TEFLONto prevent biofouling or to facilitate hydrate growth.

Expanding the focus of this disclosure back to the overall systems ofthe invention, arrays of HART modules 301, in which the porous restraintpanels support hydrate growth and function as heat exchangers, supportdesalination by promoting controlled growth of gas hydrate. Such arraysare provided in enclosures 8 that substantially or completely isolatewater being treated by the HART apparatus from open seawater, preferablyin a marine environment, so that conditions suitable for gas hydratenucleation and growth can be maintained within the enclosures 8.Nucleation and growth of gas hydrate is facilitated in part by means ofliquid refrigerant in either in a circulating system or in avaporization mode, as described above.

Furthermore, refrigerant—preferably which is also a hydrate-formingmaterial, as noted above—is advantageously utilized according to theinvention in several ways. First, the refrigerant is used to controldirectly the temperature of the water being treated. In particular,injecting refrigerant (such as liquid carbon dioxide) directly into thewater being treated within an enclosure 8 or a pressure vessel (e.g., asshown in FIG. 31), which refrigerant vaporizes and/or expands uponrelease into the seawater, cools the water to a desired temperature justabove that suitable for hydrate formation since the refrigerant absorbsheat energy from the water as it vaporizes/expands. Thus, by regulatingthe amount of such liquid or gaseous HFM released into the enclosure orpressure vessel, the desired pseudo-ambient temperature can bemaintained within the enclosure or pressure vessel. Moreover, soinjecting refrigerant that is an HFM into the water being treated helpsmaintain saturation (or even supersaturation) concentration levels ofdissolved HFM that are suitable for hydrate growth, regardless of thetemperature of source water (for common natural seawater temperatures).(Pressures required for any particular HFM to nucleate and grow hydrateare generated by immersing the enclosure 8 to an appropriate depth inthe body of water in a marine-based application of the invention or bypumping and/or vaporization within a pressure vessel in a pressurevessel-based application of the invention.)

Second, highly localized conditions for controlled hydrate nucleationand growth are created and maintained on the porous restraint panels byvirtue of their internal refrigeration. In a particularly preferredembodiment of the invention, liquid refrigerant (e.g., liquid carbondioxide) is the hydrate-forming material used in the practice of theinvention and is vaporized as part of the refrigeration process, asdescribed above. In that case, the gasified HFM produced by vaporizationof the liquid refrigerant HFM may be injected into the water beingtreated, where it contributes to hydrate growth. Alternatively, is sodesired, gasified liquid refrigerant may be recompressed andrecirculated in a vapor compression-based refrigeration cycle. In thatcase, the refrigerant in the refrigeration system is isolated from thewater being treated, and the refrigerant does not need to be the same asthe HFM used for practice of the invention (or even an HFM at all).

To better understand overall operation of the invention, it is useful tounderstand the pressure and temperature relationships in a normalseawater region where the invention may be operated and to more fullyunderstand how pressure, temperature, and concentration of dissolved HFM(CO₂ in the example used here) can be manipulated. This is because anumber of naturally occurring oceanographic conditions and physicalprocesses are utilized in the practice of the invention, whichconditions and processes are not commonly used in the manner they areused in this invention.

Many materials used as refrigerants are also hydrate-formers, includingammonia, sulfur dioxide, ethyl chloride, carbon tetrachloride,isobutane, propane, methylene chloride, chlorofluorocarbons (includingFREON), and carbon dioxide (CO₂). The solubility, vapor pressures at aliquid/gas interface, and chemical reactions of the variousrefrigerant/hydrate-formers with water (and substances normally found inseawater) vary considerably and should be taken into consideration inselecting and using a particular material as arefrigerant/hydrate-former.

Although a number of hydrate-forming gases may be used, a preferredhydrate former for use in the invention is CO₂. CO₂ has a number ofattributes that make it a preferred HFM for desalination through gashydrate using the invention. CO₂ is very soluble in water, so arelatively small volume of water can contain a substantial amount ofdissolved gas, especially under pressure. CO₂ dissolves quickly inseawater, and the seawater is thus a good media for diffusion masstransfer, which is important for the process of growing hydrate. CO₂hydrate has been shown to nucleate spontaneously and grow easily, bothexperimentally in the laboratory and in the open ocean. CO₂ is anexcellent hydrate former, with hydrate that exhibits strong rejection ofdissolved solids and saline droplet impurities, which makes CO₂ an idealHFM for desalination and water separation in general.

CO₂ is non-combustible, easy to handle, and relatively safe. Even thoughCO₂ is corrosive to certain metals at high concentrations, suchcorrosion can be mitigated through careful selection of materials. It isalso commercially available almost world-wide as a liquid that can bestored in relatively low-pressure containment vessels withoutrefrigeration, and it is available at relatively low cost. Furthermore,this already low cost is expected to decrease over time, in part becauseCO₂ from concentrated sources such as power generation is going to becaptured and disposed of in either the sea or geological reservoirsaccording to the Kyoto accords in order to mitigate global climatechange (warming). Thus, disposal of CO₂ may actually render CO₂ used inthe desalination process a profit item rather than a cost item. Inaddition, concentrated CO₂ is a strong biocide, so little biofouling islikely to occur within the apparatus itself (although its dispersal inthe ocean as part of the disposal process may quickly render theeffluent innocuous to biota). Furthermore, CO₂ is an excellentrefrigerant because of its current low cost, availability, non-toxicity,and biomedically benign nature.

CO₂ is used in this invention both to reduce the pseudo-ambient watertemperature (so that hydrate will nucleate and grow) and to provide theHFM that will combine with water to form the desalination-effectivehydrate. Thus, in the most preferred implementation of the invention,CO₂ infusion into the water being treated and CO₂-based chilling—due toboth infusion/vaporization/expansion of CO₂ in the water being treatedand refrigeration in the heat-exchanging restraint panels—take place aspart of the same overall inventive process. In order for CO₂ hydrate tonucleate and grow at a suitable pressure-depth region, the seawater mustbe supersaturated with HFM at the given pressure and the temperature ofthe water being treated usually must first be reduced from its ambienttemperature to a required lower temperature. Injecting liquid CO₂ (LCO₂)into an enclosure 8 or pressure vessel at a temperature above theliquid-to-gas phase transition in pressure-temperature space (hereafterreferred to as “liquidus”; FIG. 20) will cause violent vaporization anddissolution of the LCO₂ into the water being treated. According to theinvention, the rate of injection of CO₂ is controlled so that all of itdissolves into the water being treated, even though a transient phase ofvery tiny bubbles may exist from time to time, which bubbles willdissolve rapidly. An ideal state may be reached when a few very smallgas bubbles coexist with supersaturated water, with the remainder of theinfused CO₂ having been dissolved to the existing physical limits of thesystem. Thusor even supersaturation), apparatus within the enclosure orwithin the water being treated to specifically measure the concentrationof dissolved CO₂ is not required. Rather, the existence of a small,exsolved gas head in the enclosure 8 or pressure vessel (not shown) inwhich the invention operates is sufficient to indicate supersaturationand hence the existence of favorable hydrate growth conditions.

Although source seawater (or other water being treated) can berefrigerated or pre-chilled elsewhere by heat transfer with acirculating chilled fluid or other refrigeration apparatus, as notedpreviously, the invention preferably uses liquid HFM tochill/refrigerate the water by direct vaporization within an enclosure 8or other pressure vessel. Vaporization of LCO₂ is endothermic as afunction of the change of state of the material. FIG. 18 is a graph ofthe heat of vaporization of CO₂ for different water depths using anassumed ambient seawater temperature of about 15° C., which isapproximately the temperature of low-latitude ocean water masses at adepth of about 300 meters. (For example, FIG. 19 shows an actual,measured hydrothermal profile from near the Canary Islands in theeastern central Atlantic Ocean.) The heat of vaporization data allowsone to calculate the amount of liquid and gaseous CO₂ that must beinjected into the water to obtain a desired water temperature within anenclosure from any starting ambient water temperature. For any watertemperature/depth profile, a control diagram that dictates the amount ofliquid HFM that must be vaporized to cool the source water of any normalocean temperature to the required hydrate-forming temperature within anenclosure may be calculated and plotted.

FIG. 20, which is a plot of the pressure/temperature field of CO₂hydrate stability (the speckled region 481) illustrates the process ofcooling the water being treated and then the effect of chilling theporous restraint panels to cause gas hydrate to form on them. Inparticular, hydrate is stable to the left of the phase boundary, whichseparates the pressure/temperature field of hydrate stability (speckled)from the field to the right of the phase boundary, in which hydrate isunstable. (Concentration of the dissolved HFM is also an importantfactor to understand and consider, but for purposes of this discussion,the water being treated may be regarded as supersaturated during bothhydrate growth and dissociation.) In general, according to theinvention, ambient-temperature seawater at temperature Point A (assumedfor this discussion to be about 15° C.) is brought into an enclosure 8containing an array of HART modules, which are configured as describedpreviously. When water within the enclosure is injected with LCO₂ at apressure above (as shown on the phase diagram) the CO₂ gas/liquidtransition 483 for a range of temperatures, the LCO₂ vaporizes and coolsthe seawater to point B. (Depending on initial pressure, the line alongwhich temperature decreases will be approximately horizontal.) Thetemperature at this point is closer to being suitable for hydrateformation, but it is not quite cold enough for hydrate to nucleate andgrow. (For increasingly higher starting water temperatures, increasingamounts of vaporization are required to cool the water being treated.)

Under operational conditions, the temperature of the seawater after LCO₂is vaporized by being injected into it is monitored and the rate ofdissolution of CO₂ is controlled to maintain temperatures just above thehydrate phase boundary and to compensate for changes in the source watertemperature. Further vaporization of LCO₂ beyond that amount will simplyincrease the amount of CO₂ in the water and lower the temperature of thewater throughout the enclosure to within the field of hydrate stability,thus causing hydrate to form generally throughout the water mass whereit is not wanted.

Subsequently, further cooling/refrigeration provided by the porousrestraint panels will lower the temperature of the water being treatedfrom Point B, which is the water temperature within the enclosuregenerally, to Point C; the temperature at Point C is the temperature ofwater being treated immediately at the surface of and adjacent to theporous restraint panels (i.e., on the walls of the tapered pores) and iswithin the field of hydrate stability. Therefore, hydrate will form andgrow within the pores of the restraint panels upon further cooling ofthe water being treated from Point B to Point C.

Once sufficient hydrate has formed within the pores of the restraintpanels to clog and pressure-seal them, pressure within the narrowchamber 319 is reduced, e.g., by pumping. As noted above and explainedin further detail below, the portions of hydrate that are exposed tothat reduced pressure—ideally, hydrate that is protruding into the lowerportions of the bi-cone tapered pores, i.e., the portions of the poresthat are formed within the lower restraint panel sections 353—willdissociate back into relatively pure water (i.e., converted water) andgaseous HFM, which pass into the chamber 319. From there, the convertedwater 339 and gaseous HFM 330 (FIGS. 3 a, 3 b, 4 a, 4 b, and 5) arewithdrawn from the HART modules 301 and handled according to furtherprocessing steps as described in more detail below.

In addition to the advantages of using CO₂ for hydrate-baseddesalination enumerated above, a further benefit of using CO₂ (LCO₂) forhydrate-based desalination in a submerged apparatus is that itstransformation of state between liquid and gaseous forms takes place atpressures that are encountered at relatively shallow depths in the seaor other bodies of water. The pressure/temperature region of the CO₂hydrate stability field, in which LCO₂ hydrate-based desalination ispracticed, is within the field 481 of hydrate stability above theliquidus 483 (FIG. 20). (The higher-pressure region of hydrate stabilitybelow the liquidus will have much slower hydrate growth rates associatedwith it.) Thus, the pressure required for CO₂ hydrate to form may beachieved relatively easily by positioning an enclosure 8 with HARTmodules 301 arrayed therein within this range of water depths, and thencontrolling water temperature appropriately.

Regarding the conditions for hydrate growth, in order for hydrate growthto be maintained on the porous restraint panels, CO₂ should be kept atsupersaturation levels in the seawater in the enclosure. Ideally, thisis accomplished without lowering the overall water temperature in theenclosure (due to LCO₂ vaporization) to such an extent that hydrateforms generally throughout the enclosure, as alluded to above. This isbecause residual water—i.e., enhanced salinity brine within theenclosure that is “left behind” as hydrate is formed and extracts purewater from the seawater—will eventually be discharged from theenclosure, and hydrate that forms generally within the water mass willtend to be lost with that residual water, thus wasting HFM and loweringefficiency of the inventive process. Therefore, one or both of twosolutions to this concern may be implemented in order to maintain thewater being treated at the desired temperature and concentration.

In particular, warmer water from either greater or shallower depths maybe introduced to the enclosure as the ambient water to be treated, or itmay simply be mixed with ambient water immediately surrounding theenclosure. This facilitates further vaporization of LCO₂ into the waterin the enclosure, and hence increases CO₂ concentration, withoutlowering the temperature of the water being treated so far as to causeunwanted hydrate to form generally within the water mass. (Admittedly,balancing concentration and temperature by using seawater of differenttemperatures may ultimately prove difficult, and even thoughconcentration may be raised in this manner, it ultimately may not beraised enough or sufficiently regularly to provide for a continuousdesalination process.) Alternatively, instead of using all liquid CO₂,some gaseous CO₂, which when dissolved does not produce the same levelof chilling as vaporization of LCO₂, may be mixed with the chilled waterwithin the enclosure (with due regard for the energy balance of simpledissolution from the gas phase).

Furthermore regarding maintaining conditions for hydrate growth,detailed and on-going monitoring of local oceanography in the vicinityof the desalination installation should be conducted using standardoceanographic instrumentation and techniques so that any balance betweeninfusing LCO₂ and dissolving gaseous CO₂ can be managed in order to keepwater temperature within the enclosure in a desired narrow range, i.e.,at about Position B in FIG. 20. Such monitoring is particularlyimportant if water having different temperatures and/or drawn fromdifferent depths is used as part of the inventive process, since thesevariable temperature measurements provide the basic control data andinput data for the operational algorithms used to control thedesalination process of the invention. Additionally, diagrams similar toFIG. 20 must be generated for each installation location so that theparticular depth of the installation and the temperature of the water atthat depth (or other depths where water intakes are to be located) canbe taken into account for determining refrigeration requirements of thewater to be treated.

Turning now to a somewhat more macroscopic discussion, according to theinvention, seawater is brought into an enclosure 8 (or pressure vessel),where it is charged with HFM while its temperature is reduced to nearthe point that hydrate is stable at the given pressure depth (asexplained above). Infusion of the HFM into the source water, with itsresulting vaporization and concomitant chilling of the seawater, iscarried out entirely within the enclosure (or pressure vessel).Furthermore, according to the invention, the seawater within theenclosure may be subjected to either continuous or periodic infusion ofHFM (but on a continuous basis) so as to keep HFM concentration highenough to permit hydrate to crystallize. As hydrate grows on the porousrestraint panels and the water contained in the hydrate (i.e., thecaptured water) is extracted through the restraint panels and into thechambers 319 in the HART modules, more seawater may be brought into theenclosure, where it mixes with residual enhanced-salinity seawater(brine) remaining in the enclosure while it (the replacement seawater)is being charged with HFM. In one mode of operation according to theinvention, it is intended that hydrate be grown until the salinity ofthe residual seawater within the enclosure increases to a target level,which target level is determined by a number of factors that aredescribed in more detail below; at that point, all of the water withinthe enclosure may be allowed to exit from the enclosure so as to bereplaced by new ambient seawater from immediately outside the enclosure.Alternatively, in another mode of operation according to the invention,additional water to be treated may be introduced into the enclosure on agenerally continuous basis (either intermittently at a steady rate orconstantly) and residual brine is evacuated from the enclosure generallyat the same time, i.e., without allowing residual salinity within theenclosure to rise to a level that is too high. Thus, infusion of HFMinto the water being treated and its incorporation into hydrate aregenerally continuous processes, whereas refreshing the water to betreated in the enclosure and evacuating the enclosure may be eitherbatch or continuous processes.

As hydrate grows on the restraint panels and extracts fresh water fromthe source seawater, the salinity of the residual water (brine) withinthe enclosure increases. Additionally, the residual water will be colderthan the surrounding ambient seawater. Therefore, the density of theresidual water within the enclosure will be greater than the density ofthe surrounding ambient water so that when it (the residual water) isreleased from the enclosure 8 essentially as a bolus, the residual waterwill automatically sink downward and away from the enclosure. Thisnatural movement of the residual water away from the enclosure removesthe more saline water from the location of the desalination operation,into the lower open ocean depths and away from the surface where itcould otherwise have a larger impact on the marine biosphere. (Tofacilitate delivery of the residual water to depth, it may beadvantageous to exhaust the residual water from a pipe (not shown)extending downwardly from the desalination apparatus.)

As the residual water sinks, it mixes slightly with normal seawater,which mitigates the higher salinity and the lower pH of the residualwater caused by high levels of HFM (CO₂) dissolved in it. Additionally,as the bolus of residual water sinks, pressure on it increases and therelative level of saturation with HFM (CO₂) decreases. Therefore, theresidual water becomes undersaturated with CO₂ as it sinks so thatenvironmental effects of the initially high saturation decrease withincreasing depth.

(This dynamic removes CO₂ from the near-surface environment, in which ithas been used for desalination/water purification, and sequesters it inthe lower ocean depths. As the bolus of residual water continues tosink, pressure on it increases and temperature decreases. Therefore, CO₂hydrate may form in the residual water. Formation of CO₂ hydrate willfurther increase the overall density of the residual seawater mass andexpedite sinking of CO₂ toward the seafloor because CO₂ hydrate isnegatively buoyant. Thus, in addition to providingdesalination/purification of water, the invention provides an elegantmeans by which carbon dioxide may be sequestered.)

With regard to providing LCO₂ to the enclosure 8 for operation of theinvention, there should be little, if any, energy cost required to pumpthe LCO₂ to the depths at which the system will be operating. Inparticular, the pressure in the holding tank (e.g., 32, FIG. 1) will bethe vapor pressure of carbon dioxide at the temperature where the vesselis stored. The pressure at the bottom of the supply line to theenclosure (e.g., 36, FIG. 1) will be equal to the vapor pressure in thestorage tank plus the head pressure caused by the weight of the LCO₂ inthe delivery pipe. That pressure will, in most cases, be higher than thewater pressure at depth. Therefore, the liquid carbon dioxide can bedelivered to the hydrate-forming region in the enclosure 8 simply byopening a valve. (For liquefied HFM other than LCO₂, which may not havesimilar vapor pressure, it may be necessary to pump the HFM from astorage vessel on the surface to the enclosure.)

Once the LCO₂ reaches the enclosure 8, there are two approaches toinfusing it into the water being treated according to the invention. Ina single compartment enclosure, LCO₂ infusion, mixing, and hydrategrowth all occur within a single compartment. In a multiple compartmentenclosure, in contrast, infusion and mixing take place in one chamber,while the hydrate is grown in a separate, principal chamber that housesan array of HART modules. In either type of enclosure, infusion can berelatively turbulent, which promotes rapid dissolution of the LCO₂ intothe water being treated.

FIG. 21 a illustrates a single-compartment embodiment of an enclosure 8in which LCO₂ 502 is introduced through a manifold 505 that equallydistributes it to injectors 514 located between the HART modules 301.All operating apparatus is contained within the single compartment ofthe enclosure. LCO₂ injection is controlled by a variable valve 516,which responds to electronic control apparatus and built-in sensors (notshown). Because LCO₂ injectors can become clogged with CO₂ hydrate (oreven water ice if located outside of the CO₂ hydrate stability zone), itis recommended that the LCO₂ be injected by a manifold at a sufficientnumber of injection points that the chilling potential of the LCO₂vaporization is not over-concentrated at too small a number ofinjectors. In this embodiment, source seawater 530 is taken into theenclosure generally in the upper part of the enclosure, andenhanced-salinity residual water 535 is generally expelled from thelower part of the enclosure.

In this embodiment of the invention, source water that is untreatedexcept for charging with LCO₂ provides the media for hydrate formation.Water extraction take place through the porous restraints of the HARTmodules until the salinity of the residual water in the enclosurereaches a desired level (for instance, twice normal salinities, or about64,000 ppm). At this point, source water may entirely replace thetreated water within the enclosure until near-normal salinity seawaterone again occupies the entirety of the enclosure. Alternatively, intakeof new source water can be slower and essentially replace only thatwater extracted through hydrate formation. This mode of operation willkeep the water being treated within the enclosure relatively saline butwill keep supersaturation at a nearly constant level over time. Ineither case, the CO₂ saturation of the water under treatment within theenclosure is maintained at high enough levels so that significantdissolution of hydrate on the porous restraints does not occur.

Alternatively, FIG. 21 b illustrates a multiple-compartment embodimentof an enclosure, in which embodiment LCO₂ is infused into water beingtreated in chambers 610 that are separate from the principaldesalination chamber 612, which contains the HART modules 301. Afterinfusion in the chambers 610, the CO₂-infused water is injected into theprincipal chamber region 612, where CO₂ hydrate is grown. Infusionchambers 610 may be provided on all four sides of a rectilinearmultiple-compartment enclosure. Alternatively, if the enclosure 8 isrounded or oval, infusion chambers may be provided around the entireperiphery of the chamber 612. Still further, the infusion chambers 610may be completely separate from the enclosure 8 holding the array ofHART modules. The preferred configuration, however, is as shown in FIG.21 b.

In this embodiment, source seawater 630 is brought into the infusionchambers 610 by inlet pumps 632, and LCO₂ 502 is brought into theinfusion chambers 610 and infused into the water being treated bymanifolds 614 carrying a multitude of injectors. Provision may also bemade for a trickle of un-infused water to be brought into the upper partof the enclosure 641 so as to balance pressure within and on the outsideof the enclosure. Water movement within the multiple-chamber enclosuremay be controlled by circulation pumps 637, which withdraw water from acollection manifold 639 in the primary desalination chamber 612 andimpel it into the lower part of the infusion chambers 610. (Small arrowsindicate the general direction of water movement in the chambers of theenclosure.) The circulation pumps 637 are provided only in part to impelthe water within the enclosure, since the infusion process itself, whichcauses tiny gas bubbles to develop, will cause the water within thechambers 610 to rise. Therefore, the circulation pumps' primary functionis to control the direction of flow in addition to causing flow, sincein alternate modes of operation (which might promote better mixing ofwater and HFM), the circulation pattern could be reversed.

An infused-seawater manifold 645 injects the cooled water—which has beeninfused with HFM to appropriate levels of saturation—into the array ofHART modules 301 through a system of injectors 649 so that hydrate formsin the pores of the restraint panels of the HART modules. Conditionsbetween the HART modules 301 in a multiple-chamber enclosure aredifferent from those in a single-chamber enclosure. In particular,whereas mixing conditions within the one chamber of a single-chamberenclosure are extreme because of the direct injection of LCO₂ into thewater being treated, mixing conditions in the primary desalinationchamber 612 of a multiple-chamber enclosure are much less turbulent, andwater flow and water flow rates can be maintained as generallycontinuously downward. When the residual water within the principalchamber 612 of the enclosure has reached its desired salinity, it isexpelled 535 generally from the lower part of the enclosure so that itcan sink away from the enclosure. With this particular configuration ofthe enclosure, the seawater may be circulated through the infusionchambers and the primary desalination chamber 612 several times in orderto reach the desired level of salinity and to utilize as much HFM aspossible prior to expelling the increased-salinity water. This enhancesefficiency of the inventive process.

More specific growth and dissociation dynamics will now be described. Inparticular, fabricating the porous restraint panels in two or moresections allows the tapered pores to have complex shapes—e.g., bi-coneshapes, as noted above—that dramatically improve the hydrate formationand dissociation process as compared to that which is possible with asimple conical profile as per my previous patents. FIG. 22 a illustratesan ideal case of hydrate formation in the upper section 353 of a bi-conecomposite porous restraint panel, with hydrate 765 growing in the poreson the upstream side of the composite restraint panel. Tapered pores 714in the upper panel section 351 generally have conical cross-sections,with wide mouths 717. (See, also, FIGS. 12 a and 12 b.) These pores inthe upper panel section provide the hydrate formation region in whichhydrate growth is promoted. The lower, outlet ends 723 of the taperedpores of the upper section 351 of the porous restraint panel are smallerthan their mouths 717. In the lower restraint panel section 353, asimilar geometric relationship of the shape and disposition of thetapered pores exists in that the mouths 729 of the pores 731 in thelower panel section are larger than their necked exits 779.Significantly, the diameter of the exits or outlets of the upper sectiontapered pores is smaller than the diameter of the mouths of the lowersection tapered pores. This provides the stepped or non-monotonicallydecreasing profile that may be considered characteristic of a bi-coneprofile. The relative proportions of the diameters of the two ends ofthe tapered pores in the upper restraint panel section may vary, as maythe ratio of the lengths of the longitudinal axes of the tapered pores.

FIG. 22 b illustrates an alternate embodiment in which the pores of thelower restraint panel section 353 have larger volumes than in theembodiment illustrated in FIG. 22 a. The larger volume aids dissociationof the hydrate, which dissociation is indicated schematically by therough edges on the hydrate masses extending into the dissociation region731 (i.e., the lower pores). The larger volume allows fragments ofhydrate which may break off during the dissociation process to haveadditional space in which to reside, away from and out of contact withthe main mass of hydrate 765. (This is beneficial as it further enlargesthe surface area of hydrate and aids dissociation and more rapid waterproduction.) In this example, a rectangular profile of the lower poresis illustrated, but the actual shape may vary considerably depending onspecific materials, fabrication methods, etc. that are employed. Thelarger pore size in the lower restraint panel section facilitates betterseparation of the water and gas following dissociation of the hydrate,and it permits more pore drain holes 780 to be provided to facilitatepassage of the water and gas into the internal HART module chamber 319.Furthermore, laterally spaced pore drain holes 780 as shown in FIG. 22 bare advantageous when the porous restraints panels are verticallyoriented during operation of the HART apparatus since gas and water thathave separated after hydrate dissociation will tend to drain better thanif there is just a single, centrally located drain hole. Also, if thereis more than one drain hole for each dissociation region (lower pore),blockage of one or even more than one drain hole (e.g., by the rareoccurrence of debris or sediment trapped within the hydrate) is lesslikely to impede overall hydrate dissociation, gas/water separation, andproduction of purified water. Furthermore, in this embodiment, there isa larger refrigerant supply gallery 399, which aids distribution andoverall chilling potential of the refrigerant.

Where seawater being treated passes slowly across the face of a porousrestraint panel, a boundary layer in which lamellar flow occurs mayform. The primary mechanism by means of which dissolved hydrate-formingreactants can cross through such a boundary layer from the seawater tothe growing hydrate is diffusion, which may be an important factor thataffects the rate of hydrate growth and purity depending on the boundarylayer thickness. Therefore, the thinner the boundary layer, the shorterthe distance over which diffusion can operate as a rate-controlling andpurity-controlling phenomenon. Accordingly, it is desirable to createturbulent conditions on the face of the porous restraint panels thatintroduce micro-mixing and eliminate lamellar flow boundary layerconditions, which of keeps pumping costs low and hydrate growth kineticsrelatively high. Turbulence caused by water movement in the vicinity ofthe hydrate growth fronts enhances both the supply of dissolvedreactants—required for continued growth—to the hydrate and removal awayfrom the hydrate of material (dissolved ions, sediment, etc.) that hasbeen rejected by the hydrate.

Turbulence and micro-mixing can be induced by micro-roughness on theexposed faces of the porous restraint panels. Such micro-roughness canbe provided by several different means, each of which has the potentialto induce rotational vortices. According to one embodiment illustratedin FIG. 23 a, small protrusions 810 are formed between pores across theexposed surface of the upper restraint panel section, extending fromthat surface. Alternatively, as shown in FIG. 23 b, small, similarlylocated dimples or impressions 820 can be formed in the exposed surface,which dimples 820 cause propagating vortices (which have anintrinsically better mixing effect) but may become filled with unwantedhydrate over time. Therefore, protrusions may prove to be more reliableflow-mixers over time, since they can be fabricated from material onwhich unwanted hydrate will not grow—the protrusions either being formedseparately and attached to the upper restraint panel sections or beingintegral with the material from which the upper restraint panel sectionsare fabricated. However, any form of roughness element on the face of aporous restraint panel—possibly even a combination of protrusions anddimples or impressions—will be beneficial to hydrate growth.

Two additional considerations for optimizing practice of the inventionare providing the correct heat extraction potential through the porousrestraint panels (effectively heat exchangers) to the hydrate formationregion and maintaining the correct concentration of dissolved HFM in thewater being treated. Because water temperature within an enclosure 8 mayvary depending on the temperature of the ambient seawater being broughtinto it, the relative proportion of infused liquid and gaseous HFM mayvary. Ideally, temperature within the hydrate forming region (i.e.,within the pores in the upper restraint panel sections) is maintainedwithin a narrow range so that a constant level of HFM saturation in thewater being treated yields a predictable growth dynamic.

Furthermore regarding practice of the invention, hydrate may be grown ineither continuous or batch modes. In a continuous mode, hydrate growthand dissociation occur generally simultaneously, with hydrate growth atthe growth front (i.e., at the outer surface of the hydrate masseswithin the pores of the upper restraint panel sections) generallymatching hydrate dissociation within the pores in the lower restraintpanel sections. The overall process can be described as continuous whenthe rate of hydrate growth is balanced by the rate of hydratedissociation. In this case, the growth surface will appear to remainstationary, even though the constituents of the hydrate are movingthrough the pores of the restraint panels through a combined process ofphysical mass movement and migration of crystal constituents due torecrystallization of the hydrate. In a batch or cyclical mode ofoperation, in contrast, the rate of hydrate growth and dissociation rateare not at all times equal, so growth-dominant and dissociation-dominantperiods (which may overlap to some extent) will exist. For continuousdesalinated/purified water production, it is advantageous to establishas regular a period of cyclicity or continuity of growth as possible.

Turning now to FIG. 24, location C on that figure is example of apressure-temperature growth position for CO₂ hydrate that is just withinthe range of hydrate stability in a pressure/temperature field, whichposition is maintained within and at the surface of the growing hydrateat the mouths of the tapered pores in the upper restraint panelsections. (This position is equivalent to the growth position C forhydrate in FIG. 20.) The level or position of the growth position willvary with changing water pressure at different seawater depths, but thelocation of the growth position will remain about the same “distance” inpressure/temperature space from the phase boundary, in the metastablegrowth region of hydrate stability. An example of the position fordissociation of hydrate in the downstream end of each tapered pore on apressure/temperature diagram (Z on FIG. 24), which is the same as thepressure in the inner chamber 319 of the HART modules 301, is colderthan the growth position (C on FIG. 24) because the endothermic natureof the dissociation process consumes heat.

The salinity of the seawater affects the position of the CO₂ hydratephase boundary and, hence, the temperature at which hydrate will grow.Therefore, salinity should be monitored (e.g., via conductivity, etc.)during operation of the invention. As salinity in the enclosure changesduring the desalination process as hydrate extracts water molecules fromthe source seawater, the temperature of the porous restraints must belowered and the rate of LCO₂ infusion into the seawater adjusted so asto maintain an appropriate temperature for hydrate growth. In thisregard and to demonstrate this effect, FIG. 25 illustrates phaseboundaries that have been calculated for different water salinities. Thedifferent phase boundaries depicted are for A, fresh water at zero wt. %NaCl; B, average seawater at 3.4 wt % NaCl; C, super-saline water at 6.7wt % NaCl; and D, hyper-saline water at 15 wt % NaCl. In addition to thehydrate phase boundaries, the carbon dioxide liquid/gas transition (theliquidus) is shown as line E, since the pressure/temperature conditionswithin the enclosure should be maintained so that gas rather than liquidremains the stable free-phase form of CO₂.

A temperature drop is required to continuously produce CO₂ hydrate inthe porous restraint panels. The difference between the temperature ofthe water being treated generally within the enclosure and thetemperature of the water at the surface of the hydrate/seawaterinterface is maintained at about 2° C. If salinity were to rise to ahyper-saline level (represented by line D in FIG. 25), however,considerably more refrigeration would be required to maintain suchconditions Therefore, it is recommended that for each “batch” ofseawater being processed, desalination only be conducted until salinitywithin the enclosure reaches a level that is slightly more than twicenatural seawater salinity. Thus, where ambient salinity is relativelylow—e.g., in ocean water that has been naturally diluted by fresh water,such as may be found in large estuaries or adjacent to land that has hadrecent high runoff of precipitation, where salinities may be as low as26,000 ppm NaCl—proportionately more fresh water can be extracted fromthe seawater before such limits are reached. Conversely, in seawaterhaving proportionately higher salinities—e.g., as is common in confinedseas such as the Mediterranean and Red Sea, which have high evaporation(salinities as high as 48,000 ppm NaCl)—a proportionately lowerpercentage of fresh water may be obtained from the seawater before thecost of refrigeration becomes prohibitive.

Another relationship that should be accounted for in practicing theinvention is the relationship between depth (i.e., pressure) and thecooling effect of vaporizing CO₂, since that effect ispressure-dependent. Such relationship is illustrated in FIG. 26, inwhich A depicts the CO₂ liquidus (gaseous CO₂ above the liquidus; liquidCO₂ below it); B depicts the upper (i.e., low-pressure) limit for CO₂hydrate formation; C depicts the temperature of water being treatedwithin the enclosure obtained by vaporizing a portion of LCO₂ of about1° C. above the phase boundary (this will vary with pressure-depth); Ddepicts the practical upper limit of cooling vaporization obtained byvaporizing 100% of the LCO₂ required to reach a dissolved CO₂concentration required for hydrate formation (corresponding to positionC on FIGS. 20 and 24); and E depicts the phase boundary for brine afterremoving 50% (by volume) of pure water from seawater.

As alluded to above, hydrate constituents move continuously through thetapered pores during operation of the inventive process. FIG. 27illustrates the manner in which this transfer occurs. When the taperedpores are essentially full of hydrate and pressure is lowered in theinternal HART chamber 319, a pressure differential is created across thehydrate in each pore. Pressure in the dissociation regions of thetapered pores (i.e., within the pores in the lower restraint panelsections) is controlled by pumps, responding to sensors and computercontrols, so that it (pressure) approximately reaches a point inpressure/temperature space that is outside of the field of hydratestability (point Z, FIG. 24). Once pressure has been lowered on thedownstream side of the tapered pores, the hydrate in each of the poresbecomes subject to differential triaxial strain, which induced strain isbeneficial to the overall process of water production and to the purityof the converted water. The differential pressure physically pushes thehydrate against the tapered pore walls, and the resultantstrain—indicated in FIG. 27 by schematic strain vector arrows (curvedsingle-stem arrows within the hydrate—within the hydrate plugs (dotted)causes the hydrate to recrystalize through grain boundary migration anddefect and dislocation migration. Thus, Annealing recrystallizationconditions are induced by the strain. Where stress within the hydrate ishigh enough to cause fractures, the fractures are transpressional withrespect to the tapered pore sidewalls and thus self-sealing. Althoughsome of the hydrate in each pore—generally in the central area andincreasingly toward the necked-down pore exits—will move en masse, muchof the hydrate—especially near the pore walls, where strains aregreates—will exhibit mass transport through recrystallization. Thus,mass transfer of the water and gas constituents of the hydrate resultsfrom solution migration and diffusion down-pressure gradients, from theupstream side of the porous restraint panels to the downstream side.

Annealing recrystallization purifies the aggregate of hydrate crystalsin each tapered pore. Varying temperature within pores will acceleraterecrystallization. Impurities such as salt ions are displaced toward thefront of hydrate growth at the mouths of the tapered pores (which is themargin of the low pressure gradient in the hydrate), from which growthfront diffusion processes will tend to drive them back (arrow 785) intothe residual seawater. Moreover, the natural tendency in an aggregate ofcrystals is for “survivor” crystals, which are either larger or morestable, to grow at the expense of smaller or less stable “donor”crystals having higher surface energies. This process minimizes energyin the hydrate by reducing intercrystalline defects and dislocations andintercrystalline grain boundary area in the mass of hydrate. Triaxialstrain induced in the hydrate in the tapered pores enhances thistendency and thus aids the hydrate purification process.

The general direction of movement of the hydrate is indicated by thedouble-stemmed arrow in FIG. 27. As a result of this mass transfer—bymass movement, ductile flow, or recrystallization processes, all causedby triaxial differential stress having the axis of maximum stressapproximately parallel to the longitudinal axes of the taperedpores—hydrate is essentially extruded through the lower outlet ends 723of the pores in the upper restraint panel sections (i.e., hydrateformation regions) into the pores 731 in the lower restraint panelsections (i.e., the dissociation region).

Because dissociation is a diffusional surface phenomena, the larger thesurface area of hydrate which is exposed to lower pressure, the fasterdissociation of the hydrate mass can take place. The bi-cone or bi-partgeometry of the pores (FIGS. 4, 12, 22 a, 22 b, and 27) increases theconversion of water and gas from the solid hydrate to liquid water andgas over that which would be produced from tapered pores having a simpleconical form in which only a relatively small area of hydrate at thenarrow opening 779 of the tapered pore would be subject to low pressure.Because the conical tapered pore in the lower section is of greaterdiameter than the hydrate that is being pushed into it, considerablesurface area of the hydrate plugs is exposed to the low-pressure regionwhere it is unstable. FIGS. 22 and 27 show idealized hydrate extrusionsinto the larger diameter dissociation region of the tapered pores.Hydrate may essentially maintain its conical form as it is forced intothe hydrate dissociation region 731 (FIGS. 12, 22 a, 22 b, and 27). Asthe hydrate is progressively lower in the dissociation region, it isshown as being increasingly narrow owing to a longer time underdissociation conditions.

In certain cases where the starting water temperature is low enough (forinstance, 15° C. or less), it may be necessary to infuse gaseous CO₂instead of liquid CO₂ into the water being treated to avoid overcoolingthe water to below the desired temperature (for example, Point B of FIG.20). In that case, gaseous CO₂ that has been recovered from thedissociation of hydrate and/or from the porous restraint refrigerationsystem is dissolved into the water being treated. Reusing as muchgaseous CO₂ recovered from every cycle of hydrate formation anddissociation and from the porous restraint refrigeration system isadvantageous, and it is desired for the same gaseous CO₂ to be dissolvedin the enclosed water being treated over and over again. The primarylimit to the number of times a given volume of CO₂ may be used is thesalinity of the residual seawater (brine) that results from hydrateformation and growth. In particular, at some point the enhanced-salinityresidual water has to be exhausted from the enclosure 8, as explainedabove, and CO₂ that remains dissolved in that exhausted water will belost.

The amount of LCO₂ required to cool the water being treated by infusionfrom its initial ambient temperature (e.g., Point A on FIG. 20, or about15° C.) to the desired temperature (e.g., Point B on FIG. 20) may notdissolve enough CO₂ into the water being treated to reach the levels ofsupersaturation required for hydrate to form. However, simply infusingmore LCO₂ into the water being treated to reach the required levels ofsupersaturation may cause the temperature in the enclosure 8 to drop toomuch, which can undesirably cause hydrate to form generally throughoutthe water being treated, away from the porous restraint panels.Therefore, to obtain the levels of supersaturation required for hydrateto grow, additional CO₂ must be dissolved into the water being treatedwithin the enclosure 8, but without causing temperature of the waterbeing treated to fall too much. Infusing “supplemental” gaseous CO₂ intothe water being treated instead of liquid CO₂ enables one to meet thatrequirement.

FIGS. 28 and 29 illustrate a setup with which gaseous CO₂ from thedissociated hydrate 939 and/or from the refrigeration system941—preferably from both—can be captured and reintroduced (i.e.,dissolved into) water being treated. Pressures in the two systems areequalized, and the combined CO₂ is mixed in a pump assembly 943. The gasis then pumped 945 into a gas-dissolving region 948 of the enclosure8—also referred to as a “gas trap”—in which the water level 951 is kepthigher than the water level in the rest of the otherwise completelyflooded enclosure 8. It should be understood that the gas trap conceptand function may be implemented in a variety of forms other than thatspecifically illustrated. Further, the gas trap may be implemented in aseparate vessel with a connecting piping system (not shown).

As illustrated in FIG. 29, seawater circulated from the enclosure can besprayed or misted into a chamber (e.g., the gas trap 948) containingCO₂. Water is circulated from the enclosure 8 through conduit 983 via apumping system 985 to sprayers in the gas trap 948, as is CO₂ recoveredfrom dissociated hydrate (939) and from the permeable restraint panelrefrigeration system. A pump 989 on the reused CO₂ system may supplementthe pump system in which more than one CO₂ stream is equalized.Recirculated water is distributed via an injection manifold 987, fromwhich it is injected into the gas in the gas trap 948 through a sprayersystem having clog-free nozzles, a number of which are commerciallyavailable. (For example, SpiralJet spray nozzles (Spraying Systems Co,2005) are commonly used to dissolve gases in various fluids to levels ofsupersaturation much more effectively than bubbling gas through water.)Some ambient seawater 982 may also be introduced into the manifoldinjection system 987 to replace or compensate for water that has beenextracted by the process of hydrate formation and dissolution, or it maybe introduced by means of a completely separate system.

Reusing CO₂ that has been recovered from the refrigeration andhydrate-to-water conversion systems allows a maximum amount of recoveredwater to be produced from a given volume of LCO₂ supplied to thedesalination apparatus. Calculations indicate that with a system thatallows for multiple cycles of CO₂ recovery and reuse, up to about 15,000kg of water can be produced for every 1,000 kg of LCO₂ that isoriginally supplied to the desalination enclosure (FIG. 30). This yieldhas been calculated by assuming that the only loss of CO₂ from thesystem is that which occurs when enhanced salinity residual water isreleased from the enclosure. In practice, however, there will be small,non-systematic losses. The calculations also take into account dissolvedCO₂ remaining in the produced water, calculated over time duringmultiple hydrate formation cycles for an ambient temperature of 15° C.and until salinity of the residual brine within the enclosure 8 doublesfrom about 34,000 ppm to 68,000 ppm (C on FIG. 25). The actual massratio of desalinated water produced to initial CO₂ consumed will, ofcourse, depend on the salinity of the source seawater; the salinity ofthe residual water at the time it is released from the enclosure; theambient temperature of the seawater; and a host of other factors. Aslong as a normal range of oceanic conditions is encountered, however,this calculation should be accurate enough for large-scale estimation ofwater production capabilities.

Following dissociation of hydrate, recovered water will be saturatedwith dissolved CO₂, and gaseous CO₂ within the enclosure will besaturated with water vapor. Therefore, water/gas separation systems arerecommended—first at the HART modules themselves and secondly at thesurface facility 28 or 65 (FIGS. 1 and 2) to prepare the produced waterfor transport. Additional gas separation systems may also be provided atintermediate depths—i.e., between the desalination apparatus and thesurface. The pressure of gas that is “drawn off” at these intermediatepressure depths will be between that encountered in the dissociationchambers 319 in the HART modules 301 and the ambient surface pressuresat which the naturally carbonated produced water is kept. If it isdesired to reuse CO₂ that evolves or exsolves from the produced water,the CO₂ will have to be recompressed either to higher-pressure gaseousCO₂ or to liquid CO₂ and redelivered to the enclosure 8 containing thedesalination apparatus. Although this will require additional energy andincur additional expense, it will ensure that the CO₂ is not releaseddirectly into the atmosphere. However, operational economics mightdictate that some of the CO₂ derived from the produced fresh water willhave to be exhausted into the atmosphere (or processed in some way toprevent it) if the CO₂ is not reused for further desalination.

Operational energy costs associated with practicing the invention may bekept relatively low because system pressurization occurs naturally; theweight of the seawater itself at the depths chosen for each HFM providessuitable pressures for hydrate formation. Additionally, the HFM deliverysystem is largely self-pressurizing, so pumping the HFM (LCO₂) to depthis generally not required. The primary energy costs associated withpracticing the invention are attributable to pumping water and CO₂ aspart of the circulation systems described above in connection with FIGS.28 and 29 and to recompressing the CO₂ for reinjection into the system.For the most energy-efficient operation possible, the cost ofrecompression can be minimized by maintaining the pressure of thedownstream or water/gas recovery region within each HART module fairlyclose to the liquid/gas transition pressure for any particulartemperature. Furthermore, LCO₂ vaporization may also be used to compressother gases or to drive hydraulically driven pumps. It is also possibleto harness expanding gas in the fresh water production line (FIGS. 1 and2) to yield considerable energy. A continuing energy cost is associatedwith pumping water from depth to the top of the water return pipe, whichis determined by pressure in the internal chamber 319 of each HARTmodule 301.

FIG. 30 illustrates the amount of water that can be produced using aCO₂/HART system as described herein for different levels of CO₂ reuse.Operating with the highest energy costs produces the highest yield offresh water per carbon dioxide unit, while operating with the lowestamount of fresh water produced per unit of carbon dioxide used uses theleast amount of energy. Depending on the number of times the CO₂ isreused, different costs are incurred. If CO₂ is not reused at all,energy requirements for the system, as shown by line A (FIG. 30)(0 kWhr/kgal produced for re-compression), involves no recompression of thegaseous CO₂. Although this operational approach requires no energy forrecompression or pumping, it produces only 2.8 kilograms of water perkilogram of carbon dioxide used. (This might be acceptable if CO₂ wereso inexpensive that it cost virtually nothing, or in a situation where,for example, sequestration and disposal of the CO₂ were more importantthan producing desalinated water.)

If a portion of the free gas evolved upon hydrate dissociation isrecompressed and reused (in order to supplement the liquid carbondioxide in raising CO₂ saturation levels in the water being treated),which operational approach is illustrated by line B in FIG. 30, theenergy required remains fairly low while the fresh water productionratio (fresh water produced per unit of CO₂ used) increases. If, on theother hand, all of the carbon dioxide evolved from the hydrate (at thepressure of the gaseous CO₂) is recompressed and returned to the systemfor further hydrate formation, which operational approach is illustratedby line C in FIG. 30, water production again increases. This furtherincreases energy costs slightly while increasing the overall waterproduction ratio even more. Line D in FIG. 30 represents the maximumamount of water that can be produced per unit of carbon dioxide used,but with no regard for the cost of energy used to recompress the CO₂.

Other operational relate to disposal of carbon dioxide. When theproduced fresh water is brought to the surface, carbon dioxide thatevolves from it must either be collected and recompressed or released tothe atmosphere. Capturing energy from the expanding gas coupled withcarbon emission credits earned by disposing of the carbon dioxide withthe residual brine (which sinks to depth, as addressed above) may makecapture, reuse, and disposal of carbon dioxide gas evolved upondepressurization of the fresh water very economical.

The cost of CO₂ is a primary factor in the cost of the produced water,as it is the sole consumable used in the process. LCO₂ is one of theleast expensive industrial gases, especially when purchased in bulk.Moreover, the cost of LCO₂ may decrease even further, as carbon dioxidehas come to be regarded as the principal undesirable waste product frompower generation in view of its negative impact on global climate changewhen freely exhausted into the atmosphere. Therefore, politicalimperatives indicate that considerable LCO₂ soon may be available as acheap consumable to be disposed of in an environmentally acceptablemanner (e.g., deep in the oceans, where it will not contribute to theatmospheric greenhouse effect). Furthermore, a system of monetizing CO₂disposal already exists. Thus, where the CO₂-based marine desalinationprocess is used to dispose of CO₂, there ultimately may be no cost atall for the only consumable used in the process. The portion of carbondioxide recompressed during operation of the invention will be afunction of the market value of liquid carbon dioxide, energy, andcarbon emission credits. This will vary over time, and possibly from onelocation to another.

If a primary objective is to dispose of CO₂, the ratio of water producedto the amount of CO₂ used can be varied. For best water production,residual enhanced-salinity water within the enclosure is not exhausteduntil it has reached about twice normal seawater salinity. Conversely,if greater CO₂ disposal is required, the residual water may be exhaustedmore frequently, but at salinities that are only sufficiently increasedrelative to normal seawater to ensure that the water mass sinks to depthin the sea.

Finally, if the water being treated is treated within an artificiallypressurized apparatus (i.e., an apparatus in which pressurization is notachieved by immersion in a body of water), no separate enclosure (otherthan the pressure vessel itself) will be required, as the conditions forhydrate growth are maintained within the vessel.

The foregoing disclosure is only intended to be exemplary of the methodsand apparatus of the present invention. Departures from andmodifications to the disclosed embodiments may occur to those havingskill in the art. The scope of the invention is set forth in thefollowing claims.

1. Apparatus for desalinating or otherwise purifying water to betreated, said apparatus comprising: an enclosure having an inlet for theintroduction of said water to be treated and one or more HydrateAsymmetric Restraint Technology (“HART”) modules disposed therein, eachof said HART modules including one or more HART restraint panels formedas composite panels, with pores extending from one major surface of eachrestraint panel to an opposite major surface of each restraint panel,and an internal chamber formed therein, each of the HART restraintpanels having a series of cooling galleries extending internallythroughout it between the pores thereof; a first conduit arranged tosupply hydrate-forming material to the enclosure; and a second conduitarranged to remove purified water from the internal chambers of the HARTmodules.
 2. The apparatus of claim 1, wherein said enclosure issubmersible and is configured to be placed in pressure-equalizingcommunication with ambient, sub-aquatic surroundings.
 3. The apparatusof claim 1, wherein said enclosure comprises a pressure vessel.
 4. Theapparatus of claim 1, wherein the pores of the HART restraint panelstaper non-monotonically.
 5. The apparatus of claim 4, wherein the poresof the HART restraint panels have a bi-cone configuration.
 6. Theapparatus of claim 1, wherein the HART restraint panels comprise upperand lower restraint panel sections.
 7. The apparatus of claim 6, whereinthe HART restraint panels each comprise a refrigerant distributionmember disposed between the upper and lower restraint panel sections. 8.The apparatus of claim 7, wherein the refrigerant distribution memberscomprise microporous regions disposed within the cooling galleries ofthe HART restraint panels and dividing the cooling galleries into twosub-galleries.
 9. The apparatus of claim 7, wherein the refrigerantdistribution members comprise regions with drilled holes on the order ofabout thirty to about eighty microns disposed within the coolinggalleries of the HART restraint panels and dividing the coolinggalleries into two sub-galleries.